[mdd]

I have a suggestion that is off the beaten path. I have both a 77/22 & a 10/22. Although the factory magazines both look almost identical they are not. They have minute dimensional differences. I have tried the 10/22 mag in the 77/22 & vice-versa and they won't sit correctly. I'm curious if you didn't get a 77/22 mag by mistake. Take the rifle to your point of purchase or local gun store and tell them your problem. Any shop worth a hoot will have 10/22 rifle(s) and accessories. Borrow a verified 10/22 mag and try it. If it works, then buy a new mag or call ruger & they'll send you one for free. Ruger has been making 10/22's for a little while now & I doubt they are still ironing out the kinks. The fact that your other ruger magazine works perfectly strongly supports everything I've surmised here.
Best of luck & let us know how it turns out.

Also, most everybody on here owns a 10/22 or knows three people that own them. Tell us where you are & someone would gladly help. If you are in central Kansas, that someone will be me.