Lots of people have trouble with their Glock's jamming when they work the slide manually. Say after loading a magazine for the first time (with the slide fully forward), pulling the slide back and releasing. The Glock will jam - every time. If the magazine is inserted with the slide locked back and release the slide stop lever, it works smoothly like a charm.
First you need to check and make sure it's not the type of ammo you are using? Do you experience any jams (other than when you are manually cycling the slide)? Does this happen with all of your magazines? With all types of ammo?
Secondly, and this is probably the case, make sure you are fully pulling the slide rearward in one swift motion. Also, when you manually cycle the slide, make sure to pull it fully to the rear (I know this sounds obvious, but it is frequently the problem and the slide is not being pulled all the way rearward to the stops). When the slide is all the way rearward, keep moving your hand rearward as you let go of the slide so the full force of the recoil spring carries the slide forward.
If this fails to provide the answer, I'd find a reputable gun shop (preferably one with a range) to discuss this with. I say preferably with a range as most places frown on patrons chambering rounds unless in a controlled environment.
