I took my new (used) Glock 27 to the range today. I would chamber a loaded mag, pull the slide back and chamber a round. With the slide locked back , I could insert a loaded mag and release the slide and it would chamber the 1st round every time. After firing a round. the weapon would fail to pick up the next round to chamber it. I tried four different mags and two different brands of ammo. It doesn't seem to be picking up the back of the next round to chamber it. After the first round is fired the slide seems to be hanging up on the top of the next round instead of it being picked up and moved forward into battery. The gun has had some after market parts installed but I know little of it's history. Any ideas on what is going on?