Some shots don't have a lot of power. There's an audible difference in the amount of gas let out too, and a general feel of weakness when its fired. The shots that I think are normal give out a bigger "pop" sound while the weak shots give out a "floonk" kind of sound. Sometimes it doesn't have much sound at all and the blow back is weak.
In about 10 shots, I may get 3-4 that feel weak.
The ultimate measure of a man is not where he stands
in moments of comfort and convenience, but where he
stands at times of challenge and controversy.
- Martin Luther King, Jr.