how old is he/she ? basking temps are not hot enough basking site around 40 degree warm end 32ish and colder end must have a few degree graduate,
next questions with your uvb light how is it away from the dragon those lights are usless if its past 12 inch they havent got the penetration needs to be with in 2-6 inch i run mine not to close other wise will cause photokeratoconjunctivitis but much past they are useless also the have a life of 6months, then throw away just because they light does not mean its producing UV,
what size food are feeding? the gap between the eyes are the rule of measurement,
whats the ambient temp of the room, does the enclosure hold the heat well, whats the humdity like
start with the basic of basic things to isolate any problems then go from there most times its usualy temps/ stress that causes feeding problems
if its really underweight start assist feeding pinkies as there high in protein and fat to get some condition into your dragon
but i would seriously consider going to a vet 90days is to long for a dragon especially without brumation
there is a few things to consider not saying your not doing it right, but 90 days is a concern with only just coming into winter IMO