If that doesnt work, put the front of the slide against the edge of a bench (so the barrel is clear), solid board, etc, and gripping the gun tightly, give the back of your hand a couple of sharp raps. Thats always worked for me in the past. I used to run into that problem on occasion when I was using my old RCBS dies. If the bullet isnt plumb when you start to seat it, it can give a small bulge that doesnt look like anything but can cause this very problem, especially with the Glocks. I switched to Hornady dies, that have a bullet guide in the seating die, and the problem seems to have gone away.