I registerd to post a reply to this thread, but I have had a 15-22 for about a month and a half or so. I had a similar problem two weeks ago. I went to the range to try 5 different types of ammo. CCI Mini Mag, Golden Bullet, Wild Cat, Wincherster M-22, Federal Champion. All of them ran fine except the CCI Mini Mag which was doing the exact same thing as the OP's. Rounds would jam in the gap on the magazine feed ramp. I did notice the top round was almost level rather than canted upwards like they normally are. The Mini Mags were the last ones I ran that day so I figured things just might be dirty, but then I decided to use the same mags I had problems with and load them with Federal Champion and they ran fine.
One thing I noticed was the Mini Mags felt a little sticky and I think they have a wax coating or something one them, but not sure. I was thinking that possibly I got a bad batch because I have used the prior to this incident and they ran fine. I have also used CCI Tactical with not problems, it was just those mini-mags that were jamming ever 5th round or so.