My Marcel is a smaller one (3 lb adult) and I can't imagine him gaining 1/2 pound a week as a pup, lol
! He gained about an oz. a week from when I got him (at 16 weeks) to when he stopped gaining weight and stabilized toward his adult weight. Tinies can have later growth spurts.
I hate to mention this, but not gaining wt. can be symptom of iiver shunt, which is not uncommon in yorkies. Is she having any odd symptoms at all, or none?
Also, when Marcel was a pup, I fed him 4x a day until he was about 5-6 mths, then went to 3x, and now he is at 2x day. Maybe you aren't quite meeting her calorie requirements for growth, through no fault of your own. Is she eating the correct quanitity according to your bag of food? I mean, you could try adding in some canned food to get her to eat a bit more...