You should replace your 10 round mag followers with either the 9mm1 or 9mm3 followers. The 9mm3 followers are $4.00 each from www.glockmeister.com, but I'd call Glock Inc too even though they've quit making them. Also try pulling the slide to chamber a round instead of the slide lock, this may make it work in the meantime. I have three of the 9mm1 followers that you can have it you want them. The 9mm1 and 9mm3 followers are the same ones used in the standard capacity mags. The newest ones are different however. I had a Glock 19 and a few 10 rounders and had the same problems, I bought some pre bans and it worked fine then I replaced the 9mm1s with the 9mm3s, I never had any problems with the 9mm1 followers though, I just wanted the latest followers in my mags. They don't in anyway increase capacity of the 10 rounds, but they do make the 10th round easier to get in.
Email me your address and I'll send you these three 9mm1 followers.