Maybe just a breaking-in issue, but picked up my new 9c and having a heck of a time inserting one of the two magazines that came with the weapon. It slides in right up to the end, and at that point it stops. I have to push on it very hard to finally engage against resistance. It's even more difficult with cartridges loaded in the magazine. The other magazine slips in and snaps with gentle shove of the palm no problem.
Should I involve S&W or just keep cycling the magazine? I cannot see any differences between the two magazines on visual inspection.
Appreciate any help.

My full size 40 takes more force than my 9c. When it stops I just give it a little smack. FYI, both are only about 2 weeks old.

I had a similar issue with my 40c. Also dropped magazines during firing. S&W replaced the magazine catch. No problems since. Great customer service.

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Romans 8:37

If you're inserting the magazine with the slide closed download the magazine by one (1) round and see how she engages. Not something you want to do as there's one less round in the pistol, but this will determine where the problem is and we know the other magazine is OK.

Assuming there's no problem with the downloaded magazine disassemble it and remove 1/32 inch from the bottom of the follower skirts to allow the top of the follower to lower a bit more when the top round presses against the slide.

10-round magazines are especially subject to this problem because of the capacity restrictions in the Occupied States where an 11th round is very bad.

-- Chuck

Are the Occupied States where all those goons hang out and mess the place up and want you and me to pay for them to get free stuff?

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59,M&P9,40,40C,357,(2)45,G26

Chuck S, you're dead on. It turns out the first magazine (that engaged just fine) only had 9 rounds because I was having so much trouble trying to load a tenth round into that magazine. When I was able to shove a tenth round into it, it also was unable to engage just like the second magazine. Both magazines engage just fine with nine rounds.
So, I guess it's time to head into the garage and remove part of the follower per your advice (now I just have to learn how to disassemble the magazine). Thanks for the help.