I've owned my P220 for 20 years without incident. All of a sudden, the magazines will not seat correctly unless the slide is open. What could the issue be? Thanks.

Have you changed ammunition brands lately?
Ive noticed different brands make the mags a little harder to seat in my 220.

HTH...Todd

No, I load my own. They are consistent.

OAL length a hair too long?

Went through that with a P220 years ago, OAL had crept longer and I could no longer load a full mag. (didn't rear it's head till I was loading mags at a match)

If you're right at that point where you can fully load a mag, but have no extra play, that would do it. Load 8 rounds to a shorter OAL and see if it helps.

Dirty mags / spring bound up in some way.
Slide lock / release or spring broken causing issue
Mag release broken
Double check bottom (interior) of slide to make sure nothing broken there
Ammo slightly out of spec./your reloading has gotten out of spec either mechanically or you screwed up a batch (no disrespect meant just thinking of root causes and we all screw up now and then)

Dumb thought, you have weak springs in the mags and loaded 8 in a 7 rounder or 9 in an 8 rounder??? Stupid but stranger things and all.

Could be the loading. No offense taken. I'm going to try some factory ammo today. With the slide removed, it works fine. Will give the weapon a thorough cleaning as well.

Unlikely answer to your problem, but maybe your hand strength has deteriorated a little over the past 20 years?

Magazines won’t seat at all or difficult to seat? Does it seat if you give it a solid tap?

Obviously with the top round pushing against the closed slide, the mag spring need to compress just a bit more. Did you recently buy new mags? Change springs? Accidentally load an extra round on a really weak spring (not likely)?

Out of curiosity did you ever figure out what the issue is.