I had the same issue, and used the following fix. The plate on the back of the spring guide does not quite clear the under-lug of the barrel. Break the back edge of the plate on spring guide (the edge that is trying to slide past the lug on barrel) with a small fine file. Not alot just round it so it is not a 90 degree corner. Than stone it or use fine grit paper to smooth it up. Do not do any thing to the barrel lugs.
Advise you wear eye protection, and every one else in the room. If it gets away from you the spring guide will launch like a dart, and could cause some damage, punctured eye, chipped tooth, broken tv screen.
Try to get it in again, and when you reach the sticking point, give it a tap, forward not down, with a small brass hammer or similar tool. You may need a third hand for this. If it does not go on with a slight tap, don't force it.
Good luck.
It will come off easier than it goes on. r gunfrog