I'm not very familiar either but wondering if your using the same magazine all the time. Does it do it with more than one magazine?
I'd also check the spring on the slide lock, I don't recall what it even looks like but it should keep it down until pushed up by the mag.
Maybe it's broke or bent. Just tossing out ideas here with nothing to back it up with.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."