slide stop lever will not engage

I have a MC1sc with manual safety, serial#009469CP, purchased in June, 2019. With the slide retracted, the stop lever will not engage. The lever works freely with the pistol disassembled. Before anyone jumps in with the obvious, yes, I'm sure the slide is fully retracted. Anyone had this happen to them?

Naturally, I find this out one month after the end of the Limited Warranty period.

I assume you are trying this with an empty magazine or are you saying that you cannot push up the slide release without an empty magazine in place? Have you tried a different magazine?

Thanks for your suggestion. It now works! It's a mystery, because I did the things you suggested before my OP.