Slide locks back with ammo still in magazine

Today I was out shooting my Beretta 90-two and I ran into a bit of a problem. The slide locks back while there is still ammunition in the magazine every once in a while. Today was my first day shooting a new batch of reloaded shells so I'm thinking that that may have something to do with it. I have heard of people having problems with the bullet coming into contact with the slide lock if the bullet is forward at all in the magazine but this doesn't appear to be the problem in this case. Any ideas as to what the cause is would be greatly appreciated.