Ruger Mark II 22/45 magazines

I have a Ruger Mark II 22/45 and a while back I lost the two magazines that came with it, plus a new one I got late last year. The reliability was pretty good with only the occasional jam. Earlier this month, I got two brand new 22/45 magazines ($20/per) from SW and at the match today I was jamming regularly, and in fact it cost me the match.

The jam seems to occur with one fired case getting stuck behind the new cartridge on it's way into the chamber and the bolt & breech face. Jeffersonian has suggested a new extractor, which may well be it. The thing that bothers me is that with the old magazines, jamming was rare. Plus, at the range earlier this month, I borrowed a friends magazine and had jamming problems until I gave that one back to him and he lent me another one. This time it shot like a dream and I had no more malfunctions.

Is there a difference between the Mark II 22/45 magazines & Mark III 22/45 magazines? If so, what is it?

Any other ideas what is going on?

UPDATE: I should have said this earlier, but the replacement magazines I got are from Ruger.