My baby has always, and still does, the exact same thing. It is a good thing for me though, because I have an oversupply and it's as if she is automatically block-feeding. If your supply is average though, as in right for your baby's needs, then I can see how the lack of stimulation could be a problem. As long as she is producing enough diapers, you can be sure she is getting enough... But as far as the supply issue, I'm not sure. I would try gently waking her up and offerig the other breast. Maybe she'll eat, maybe she'll just comfort suck until she falls back asleep?