Dang... maybe a squib reload and shooter racked back a 2nd round instead of checking?
Maybe a blockage of some sort?
I've always received uppers that we're factory fired before shipped to me and even then I cleaned the crap out then and fired single shots for functionality
There has to be...
So your the original owner, put 400 rounds or so down range, and it won't fire now?
Please elaborate if you removed anything to give a quick cleaning like the trigger bar? Could very well be the connector spring installed upside down....
If not I'd recommend the same; take it to Glock
As another member stated a while back, when sh#+ gets real, that's when Glock shines... No DA/SA bs, just point and shoot.
General range trips are fun but real life under stress situations won't have time for playing around.... Just my 2 cents.