How heavily are you dusting the crickets? They should look very lightly frosted, not as though they have been dipped in white paint.
I know you said all your temperatures are good but please let us know what they are. Use a digital thermometer with probe for air/ambient temperature readings and surface temperatures (or a temp gun for surface, which works much better). Find the temperature of your basking spot, and the air temperature there, as well as the cool end of the tank.
What size cage is he in? Do you have a UVB source? What substrate to you have? Pictures of your setup will be great as well.
As your dragon is about 6" long, he is probably under two months old, or a poorly fed older dragon. Pet stores tend to feed their dragons very little to keep them from growing, so they are more appealing to buyers. Good reliable reptile knowledgeable pet stores don't do this as they want healthy and thriving animals to go to good homes.
Tell us more information about how long you had your dragon, how often do you offer food? How many crickets does he eat?
What sort of supplements are you using (ie what brand of calcium powder and what brand of multivitamin?)
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PHLdyPayne