The guide rod/contact spot on the lug under the barrel has to be pretty well aligned.
I hold the slide upside down, drop the barrel in and place it in the fully locked position in the slide.
Then I put the guide rod in place while insuring that the barrel stays in place and the back end of the guide rod is both fully "down" against the upside down barrel and evenly placed side to side. Once it's in that spot I've not had one move while putting the slide assembly on the frame.
I have, more than a couple times, put the guide rod in place, but with one side or the other (left or right) at the back end not quite lined up side to side and sure enough, the darn thing will hang up as I push the slide onto the frame. Sometimes it'll come right back off (if I stopped when it didn't feel quite right) but a time or two it took extra effort to get the slide back off the frame and sure enough, the back of the guide rod was off to the side of where it needed to be.
Watch, as you put the slide on, go slow and see if the guide rod starts to push out before the slide is all the way in place on the frame. If so, stop and pull the slide to check that guide rod to barrel alignment.
Good luck with it. It's another thing that "the more you do it, the easier it gets."
