Brought a Bulgarian Makarov from AIM a week ago and having problem with the slide not locking open.  The slide stop won't “catch” the slide.  When pressure is apply to the slide stop it will lock open, but as soon as lift my thumb off the slide will slam forward.  I disassembled the pistol and found no obvious damage.
It will lock open on a empty mag, I tried two different mags.  But when the magazine is remove the slide will slam forward.

What might be causing the slide stop not to engage?  Thank.

No spring under it? Or the spring isn't in the right spot?

I was having the same exact problem from one I ordered off classicfirearms. I filed down the the area where the slide stop catches the slide to make it less worn down as I think the slide was pushing the slide stop down. and it works like it should now.

Im having the same problem with my Aim Bulgarian Mak.  Could you post a picture of your modification?