any chance your grip may have loosened up. Gun recoiling could cause this if your grip is too loose. Keep wrist locked, good firm grip and see if gun continues to do so. Have someone watch you shoot and see if they can see any mechanics issues.
Have you stripped it down to check for under/over lube? I had a similar problem that boiled down to too much lube. I had no idea glock's liked to be run so dry.
Could be mechanical, check carbon buildup or wear on the extractor. How old is the spring. Gen1/2/3/4? Seems I read somewhere that early Gen 4's had spring issues, but soon fixed it by converting back to the Gen 1/2/3 setup.
Could just boil down to ammo, but Winchester whitebox would be strange. It's normally the high end/pricey ammo that gives me fits in the 17. I can't get a seamless mag of any Corbon through it, but it'll eat blazer all day no problem.
Sounds like you need to replace the extractor spring at the very least. If it has a lot of miles on it, replacing the extractor spring and extractor should do the trick. While you're at it, might as well replace the trigger spring, firing pin spring, and recoil spring.