Hi, sorry to hear that your beardie is concerning you. Usually when they spend a lot of time hiding, like under their rock, either they aren't feeling well or something in their tank needs a little adjusting. If you could fill us in on the basking & cool side temps and the type of thermometer you're using, it would help us rule things out so that we can make sure everything is perfect. A 6 yr old beardie can be particular with their needs just like a little baby can. As the seasons change, sometimes it's necessary to go up or down on the bulb wattages or distance of the bulbs to the basking area. Where exactly are you taking the temps?
As beardies get older, sometimes their bodies can't absorb the supplements the way they did when they were younger. Using liquid calcium might be something you would want to try and it's available in a variety of flavors that they really like. You can get it here: http://bug-de-lite.com/Products%20Page.html Tracie, the owner of that website is also another moderator here and has valuable info for all. She also ships her products very quickly and will answer any questions you may have. She also carries Benebac gel which is also great for boosting there appetites and they really love the taste.
Here's a site that shows the best greens to feed: http://www.beautifuldragons.com/Nutrition.html Also, as they get older their tastes change so even though they won't try one of the staple greens one wk, the following wk, they may love it. My older beardies loved escarole, turnip & mustard greens, collards and dandelion greens. Perhaps yours would like at least one of those. Sometimes adult beardies won't eat feeders every day so that's why it's a good idea to try to find a green/veggie that they like.
Chicken & squash baby food is well likes by beardies too and it's easy to get them to lick it off their noses. For a 6 yr old beardie, you should actually use more squash with a smaller amt of chicken. You can put the supplements in that too.
Also, since there are some UVB's that can cause all sorts of health issues, could you let us know what brand & type you have, what the distance is of it and how old the bulb is?
If you haven't had a fecal checked for parasites lately, that would also be a good idea just to be able to rule it out.
Please be sure to keep us updated on him and let us know if we can help with any other info or suggestions.