I have the same problem with one of my two 13-round magazines. The 13th round will load in one of them but not in the other. (I can get 11 rounds in by hand but need my two-handed magazine loader to get any more rounds in either of them.)
I decided to disassemble both magazines and see if the internals were different. They WERE! The spring in the magazine that will not allow the 13th round to be inserted was 1/4" LONGER than the other spring, plus it's also a bit over 1/16" WIDER at the bottom bends! See picture below:
As you can see, the bad spring is definitely longer uncompressed. It has much greater bend angles in all bends past the 2&1/2 inch mark, plus the bottom bend is extreme and the bend at the top does not fit the follower flush or as well as the good spring does.
Yes, if I put the "good" spring in the other magazine that had the "bad" spring in it, magically the 13th round will load. Why am I not surprised? It was clear to me that the defective spring was binding up inside the walls of the magazine and not allowing the last round to load. It was too long and too wide!
I contacted S&W CS and sent them the above picture with my complaint that the magazine with the bad spring would not allow the 13th round to load even using my two-handed magazine loader from EZMAGLOADER, below:
They are sending me a replacement spring!
I'll report back on this thread when I receive the replacement spring and give it a try.