This is a common problem with Glocks. The problem is with the extractor. The fix is to sand paper both flat sides of the extractor and to polish the round portion. Basically smooth out all surfaces that are in contact with the gun.
Here's a discussion that's lengthy, but it worked for me.
http://www.glocktalk.com/forums/showthread.php?t=1402990
Here's a discussion that's lengthy, but it worked for me.
http://www.glocktalk.com/forums/showthread.php?t=1402990