Have A new to me circa P225 . Cane with 4 Factory Sig Zipper mags.
One of them once loaded has to be forced in and Pulled out hard.
Empty IN/OUT easy
Sorry can post pic.
Thanks

The first thing I’d suspect is that one/more of the spot welds has broken and even though it’s a single column magazine, some side pressure on the magazine tube is causing it to expand when loaded. I would load the magazine full and carefully inspect the back seam to see if it’s being spread open.

When the mag is loaded, is there any deformation of the mag that you can see? Maybe one of the seams have given way/separated under pressure. Also is this the only mag that does this?

If the magazines appear to be OK, check the grip screws if they extend too far into the magazine well which will cause a long scratch to appear on the magazines.

There should be star washers under the screws.

Had basically the same problem once with a p220 mag. Found it had a very small crack in the back at the top lip. Couldn't see the crack when unloaded, but was there when mag was loaded and caused enough resistance where it would not drop free.