It would be interesting to see some of the test data for DARPA study. I remember reading Ampakins can have greater effect the older you are. It has to do with your bodies natural aging process. So, if the soldiers tested were in the 18 to 20 year old range, it may not have the same effect as a soldier in his or her 30+ age range. I don't know, just guessing here.
One day, the test data maybe released? Cortex is still interested in sleep disorders, so they saw something.