Behavior, stress, and sickness are all very broad topics but all can go hand-in-hand with each other.
When dragons are stressed, their behavior changes and it can be because of, but always isn't related to, sickness. Dragons coming home can cause issues with eating the first week or so, while they're adjusting. This can also lead to a change in behavior. The dragon isn't necessarily sick, they're adjusting and not eating or acting normally, because of it.
Dragons can also become ill and oftentimes are horrible at showing their pain. Some dragons develop a black beard depending on the degree of pain. I've seen dragons in pain with no black beard present and I've actually even had a dragon have surgery on his hand and walked on it, acting like he was in no pain at all, a few hours post-surgery.
Because dragons are oftentimes horrible at showing pain, behavior is a key indicator to discovering any possible sickness. Parasites, respiratory infections, and other issues are oftentimes first noted by a dragon acting unusual or appearing to be stressed. Females getting ready to lay infertile eggs can also appear stressed. (Note that not all adult females lay eggs, only some do, and this happens without ever having any contact with a male dragon.)
It is imperative from an overall health standpoint to determine why your dragon is constantly showing signs of stress or has unusual behavior. Anytime any concerning issue is noted that can't be properly identified by the owner, a vet should be contacted.
One of the biggest concerns I see with owners, especially new owners, is that their dragon has larger, more noticeable pores and the owner assumes they're clogged.
There is a natural waxy substance in the pores and this is more prominent in male dragons. Not all males have huge pores and not all females have small pores. I have some males where the pores are so small they're not even visible and some females with pores larger than those of some of my adult males. Side note: Stop using pores to determine gender. Lift the tail to determine a dragon's gender.
The main problem is that when an owner assumes pores are "clogged" simply because they're large and/or filled, the owner ends up doing things like soaking and scrubbing them, which does nothing but irritate the area. If pores are severely clogged, see your local reptile vet and the vet can extract the substance and clear the pores. This should not be done at home, as the pores (now empty) can easily become infected, if proper care isn't taken.
Coil uvb bulb issue
The main issue with the lighting is that back around 2007, it was discovered that the coils caused an eye issue called photo-kerato-conjunctivitis. As more and more cases started to appear in reptiles, researchers began noting that the incidence was occurring where coils were being used. Because of this, researchers advised people whose animals had this condition (which is a very painful eye condition) to turn off their coils for 1-2 days and watch for improvement. Low and behold, the animals improved after the coil was shut off.
The problem was the radiation exposure through the bulb. It also caused behaviors such as the dragons stopped basking and refused to eat normally, as well as starting to eventually develop signs of MBD. Swollen eyelids were also commonly noted through various studies.
The coil manufacturers (mainly Zoo Med) began to change their manufacturing process on the coils while continuing to sell them. It is believed that the majority of the coils that were proven to directly cause the issue are now off the shelves but that is an unknown fact that to my knowledge, has never been proven valid. If buyers had an overstock of coils, it is actually possible that the overstocks still could be being sold and used by customers. Another major sign of health effects from the coil is a dragon that is closing its eyes frequently or making a "squinting" like action. (Do not ... not ... read into this as a dragon that sometimes basks with their eyes closed, which is normal for any dragon to do. We're talking about excessively closing the eyes and/or squinting, during the majority of the time the dragon is under uvb exposure.)
Again, the issue is supposedly fixed in the new coils but that's still not proven. Zoo Med never released a statement, as they had promised, regarding the improved safety measures.
The issue with distance is that the coils can't properly reflect the right amount of light that's needed for a bearded dragon. An expert in the UK wrote a forum piece on his research. He found the following after his two-year study --
"Compact lamps are almost impossible to reflect whatever brand you use, in fact I have just had a 2 year project to find the best way of reflecting them.
As such and as you can see light is produced all the way around the lamp. As such only a tiny fraction of the light pushes downwards onto the dragon where it is needed most, in fact only about 12% of the lamps output is available downwards if unreflected properly.
As such they do not create a usable gradient when used un-reflected and should not be used as a sole light source in a big enclosure.
The good thing with linear lamps is that they are so easy to reflect had as such all of the light can be made available to the animal = vastly more efficient and effective."
He never published any of his research following that, although it was quite recent (June of 2015).
It sounds like the combined information people are passing along when they say that coils are bad is a combination of his ^^ distance research along with the research found by the UV Guide (they're a reptile research company in the UK).
Here are the supporting links -
The health issues - found with the coils but not the tubes.
The UK researcher that ran distance studies -
And the most serious and widely-known health issue from the coils, which is the eye dangers -
Case histories that were started on this page - pretty thorough. Include coils used on hatchlings all the way to adult dragons
Failure to thrive is unfortunately a condition that's oftentimes encountered as a result of various situations. Pet stores sell dragons that are sick or not thriving well on their own, and also by breeders who aren't reputable, that end up putting sick dragons into the market or dragons who simply aren't strong enough to survive on their own, as dragons should.
Dragons shouldn't have to be "taught" how to eat. Its a natural survival instinct. If you see breeders or keepers saying that they had to force feed babies to thrive, the baby isn't thriving on its own, period. Regardless of morphs or other circumstance, babies should be eating on their own from the time they start eating, after they hatch.
Over-producing dragons, weak genetics, keeping babies after they hatch in bad conditions and with improper care can all easily lead to a failure to thrive situation. Each situation must be addressed individually. Oftentimes, a baby can survive with critical care but in all reality, when the baby is a "failure to thrive," little can often be done to push the baby to live and the most humane options need to be discussed with a vet to determine if the baby should be euthanized, to save him/her from additional suffering.