Hey, what could be the reason my loaded magazine won't clamp and completely locked into the gun, but yet it locks into the gun when the magazine is unloaded?
Put the barrel/slide assembly back on, make sure its seated properly. Get the lever started in roughly the "unlocked" position, then rack and hold it all the way back (dont use the slide stop) and you should be able to push it the rest of the way in with your thumb. I'm assuming you figured this out already.