Two things to always check when you're having shocker issues, the battery and the ball detents. The battery has to be hot, preferably a duracell alkaline, I like rechargeables, but when they start to fail they fail weird. The Ball detents need to be clean and move easily, if they aren't or if it's sticky and won't push in easily, it can inhibit the bolt movement forwards which can give you bad results.
The great thing about having 2 other functional shockers is that you can simply swap components and probably very easily determine the faulty part.
I'd start with the bolt. Put the bad gun bolt into one of the guns that works. You can put the whole assembly in and try it. At the same time, take the good gun assembly and drop it into the bad gun. If it does the same thing on either gun, it'll tell you something. Work your way to the board, etc. With a spare good fun, it should be easy to figure your problem out.
But also be sure that you put the correct insert into the correct hole.