Why doesn't breastfeeding make my baby sleepy?

My daughter is almost 7 weeks old and I don't understand why breastfeeding doesn't help her fall asleep at all. With my son at this age he would always go to sleep so easily after a feeding but with my daughter she can eat and eat and eat and she's still wide awake. What's wrong with my milk that it's not making her sleepy?? It's so frustrating because we so everything to make her sleep and she won't fall asleep or stay asleep when she needs to because she's overtired all the time from not going to sleep.