After missing the one I sold not so long ago, I picked up a new one. I already had a spare set of "trijicon" sights lying around, so fitted those (as I had without issue on my previous one). I reassembled and the gun racks flawlessly without the mag in. If the mag is in but empty it will lock back after the first shot, as it should. However after that first shot the slide won't return forward, I have give it a little help. This also means when trying to actually fire it "jams" with every shot, the slide sticking half-way.
I've cleaned and lubed, I've tried various hop buckings. I've even changed the nozzle but I'm stumped. I'm sure it's nothing to do with the lower half so that only leaves their being an issue with how the slide is formed (possible warping etc) or the BBU housing itself as the nozzle springs forward just fine.
While I'm no tech, I like to think I know more than the average skirmisher when it comes to getting Glocks to work, so I just thought I'd put this out here on the off chance anyone had any suggestions. If not I'll order a new BBU and see if that solves it.