So the issue is getting the spring guide stopper to go back into the trigger box which is attached to the receiver/cylinder?
If so, sometimes the cylinder unit rotates while in the receiver. If you look at your cylinder, you will notice that there is a long gap and you can see the spring and spring guide.
When trying to get the spring guide stopper into the trigger box the cylinder gap needs to be presently facing towards you, so that the spring guide stopper can push up through that gap. If the cylinder is slightly slanted and only part of the "gap" is showing, the spring guide stopper will not go back in.
Simply take an allen wrench to the cylinder and rotate it so that the gap is fully visible and the spring guide stopper should slide right in (through trigger box, connecting itself--here a "snap" noise.)
I think that is what you are referring to, am I right?