Those already with an R9 may have found this out ..... but when reassembling I found one thing in particular which initially seemed to make things difficult.
Inspection and thought soon came up with reason and solution is simple. Look at pic below (borrowed from DDgator's post) ..... you'll see at the inner (left) end of the spring assembly - there is a circular disk ... which impinges on the barrel lug. In order to easily refit the slide, this disk should be parallel with slide in both plains.
If slightly ''off'' then that disk will not return into the close fitting area on the frame and you'll find that as you try to move the last 1/2", the spring (springs) come under compression, early. So - just eye up the assembly nicely parallel first off .. and should be no probs.