Do you perform a half hitch after each bunch of hair is tied in and spun and packed?
Do you cement the half hitch?
If neither one of these are done, then the packed deer hair may start to creep up the shank as the fly is tied, resulting in an eventual loss of tightly packed material.
The only other thing I can say is, if you think you can't fit any more hair on the hook, use your packing tool and fit some more.
You're probably already doing these things, but that's all I can really think of off the top of my head.