Jesse, sorry you are having this issue. We based our answers on what S&W says about the FPC, that it can accept M&P full-size and compact pistol double-stack magazines. I would reach out to S&W at 1-800-331-0852.

Jesse, doesn't sound normal. Like Trevor said if the magazine is full of ammo when this is happening then try loading the magazine to 8-9 rounds instead of 10 until the magazine is broke in. Alot of new magazines have trouble being seated properly when fully loaded, they're made that way on purpose though and once the spring loosens with use it'll function properly. If the magazine is completely empty and the magazine catch still isn't catching properly the magazine may be a manufacturers defect because according to Smith and Wesson the fpc will take full sized and compact M&P magazines. If that's the case your only option other than altering the magazine is to contact Smith and Wesson at 1-800-331-0852
Monday-Friday 8:00AM-4:45PM or here's a link you can use to email them. Sorry you're having trouble but hope this helps some, link to email here................. smith-wesson.com/customer-service/contact-us#emailform

Is it difficult to insert with the magazine empty or is it fully loaded? If it is empty there may be an issue worthy of a return, but loaded magazines are commonly difficult to seat when the top round is resting against the bottom of the bolt on a large variety of firearms. If that's the case it is often much easier to insert a magazine with the bolt locked open. Even with the bolt closed they often still function as intended if you give the bottom of the magazine an authoritative tap to insert fully.

Hi Jesse, this mag should fit S&W M&P9C (compact) pistols. For the Smith and Wesson FPC I use : budsgunshop.com/product_info.php/products_id/32911/smith+%26+wesson+10+round+black+magazine+for+m%26p+9mm - S&W does state the FPC is compatible with M&P® full-size and compact pistol double-stack magazines. You may have an out of spec magazine so call S&W Customer Service at 1-800-331-0852
Monday-Friday 8:00AM-4:45PM EST

Hi Jesse, this mag should fit S&W M&P9C (compact) pistols. For the Smith and Wesson FPC I use : budsgunshop.com/product_info.php/products_id/32911/smith+%26+wesson+10+round+black+magazine+for+m%26p+9mm - S&W does state the FPC is compatible with M&P® full-size and compact pistol double-stack magazines. You may have an out of spec magazine so call S&W Customer Service at 1-800-331-0852
Monday-Friday 8:00AM-4:45PM EST