I got a Glock 43, brought it hope, functioned it, and took it apart to polish the trigger parts, install a different front sight, and reverse the rear sight.
Now the trigger fires when pulled, but also, while holding the trigger back and after cycling the slide and then:
- when the slide is cycled other than gently
- when pressure is eased up on the trigger slightly (e.g., to begin to reset)
- when the gun is shaken
I am almost certain it's the sear engagement, the back of the cruciform with the protruding piece of the striker. (My orange plate for checking engagement on full size guns doesn't fit.) Nonetheless, I bent the connector every which way before giving up.
By polishing I mean lightly, nylon brush on a buffer wheel and Dremel. No stones, sandpaper, polishing compound, or even toothpaste.