Probably This is the most common problem in KSG Glock lines. Even a new one could face this problem. As far as I know there are several problem can cause this. Here steps to identified:
here’s the diagram: link
1. Check parts: #23, #27
Make sure spring still good, no tear or deform piston head
2. Now check nozle (#22) make sure no small crack.
if al 3 parts above still good, now try this:
The problem come from part #19 in G17 or #251 in G18c, it’s wear out so cannot push hammer correctly.
The pin #21 G17 that hold that part can become loose and didnt stay in the midle can cause this also.
And this is how I have it fixed:
1. Fixing the loosen pin, just scratch one end of that pin, so it wont loose again
2. If part #19 in G17 too wear out maybe you should replace it (cost $5 ). But you might want to try this first:
Put that paper inside and assambly
And my ksc glock 17 working again. Cycle normally.
Hope this work for you.