The first thing you should do is look to see if the primer strikes were good. Take the casings that fired and compare those to the rounds that did not fire. Do the indents on the primers look the same. If so, you might have bad ammo. If not, it could be that the ammo you were using had "hard"...
First post, but I do have a pretty good amount of experience here. I know this is not in the location you specified, but I would take it to Scott at Jim's Firearms in Baton Rouge. Very good work and very professional.