First, not to be unnecessarily picky, the part I am describing is the Slide Stop Lever, not the Slide Lock.
Originally Posted by Gooch
You have the wrong Slide Stop Lever in the gun, part number SP 05306. Here's the description for that part:
" Slide Stop Lever & Spring - fits only G20-G20SF-G21-G21SF(standard mag catch)-G21 Gen4 -G29-G29SF-G30-G30SF only (marked 5299-1 on back).
That part is for 10mm and .45ACP guns. I'm pretty sure that the -2 suffix stamped on yours is not significant.
The part you should have is 2919, here's the description:
"Slide Stop Lever & Spring - fits 9mm, .40, .357 (NOT 2-pin G17/G17L/G34) (marked 2912, 2912-2, 2912-3), - including Gen4's".
You can buy the right part here: Slide Stop Lever & Spring fits most 9mm - 40SW, 357SIG-www.glockmeister.com
It's entirely possible that this is the entire cause of the problem. I would replace the part before pondering this much more.
That said, I would follow MtStream's advice on testing the Recoil Spring Assembly, that needs to work properly. Now that we have found one incorrect part replaced in the gun, there may be more.