Just used a brand new magazine for the first time today for my Sig Sauer P365 and it was able to fire 10 rounds twice (10 round capacity). Then, I went to insert 3 bullets into the magazine, they went in with a bit extra force needed, and the fourth bullet WOULD NOT go in at all. Any idea why this would be happening? I took out the spring and compared it to a similar functioning magazine & I couldn't see any defects/damage.

I'd really appreciate any helpful assistance.