Could be many issues. Are her temps/humidity levels correct? What type of veggies/insects is she being fed? Has she been checked for parasites?

She might be dehydrated for one. Does she get soaked every week? My 1 year old male was pretty similar to what you are describing. I took him to the vet. He did not find anything wrong with him other than a light parasite load. He did deworm him after I begged (my vet hates to use meds unless absolutely necessary), but it still took a while for him to get back to eating on his own (I did give him baby food w/ TRex veggie dust & calcium about 3 times a week because I was afraid that he'd get worse w/o any nutrition). I think the Trex veggie dust did help.

I'm sure more experienced Beardie keepers will chime in soon also.