With brachys at that size I have found that keeping them a little more wet then the adults is not a bad thing...... also make the container alot smaller. maybe hes having a hard time finding and chasing things you put in there for food.
^ I agree w/ talkenlate04, my Brachy slings (vegans, albopilosum, smithi) are kept on slightly moist peat on one side of their vial and all are molting and eating. Give that a try and like mentioned if your enclosure is too big try keeping it in a smaller container, diameter preferably not bigger than twice the legspan.
I agree with the above and I'll add that Brachypelma spp. and alot of your desert dwelling species will go off feed for no reason. All I can say is keep your Brachypelma klaasi well hydrated and offer food to it every once in a while and it should be fine.
Such things readily happens with some species like Brachypelma spp, Aphonopelma spp. and Grammostola spp. (especially G. rosea and G. pulchra).
If You want the sling to grow and molt faster (sooner) You should made contrastly different conditions... just it... but sure these conditions should be in "reasonable limits" for this species.