Exactly what do you expect him to do? He doesn't have the power of indictment, and he doesn't have the power to enforce subpoenas when the Administration doesn't comply. On top of that, we all know what Hillary did to conceal her communications.
I've seen federal prosecutors do this before. They are methodical, almost annoying so. It takes what seems like too long, and then they release everything in one huge blast, and the reaction tends to be "wow - I didn't know they had all that."