That has been a common problem. I just slightly opened the loop very gently with a flat headed screwdriver and it stayed on after that. It can be maddening. Generally, I avoid this issue by not removing the grips, if possible.
I've been occupied with searching for that elusive "controlling precedent" on a client's case and just noticed this thread.
Tracker nailed it on the solution to that little problem which occasionally arises. The tolerances of the R9 are minimal, including the fitting tolerance of the trigger bar spring with its post. I first used the technique Tracker describes some years ago, and I now remove the grips of my R9 every time I clean it -- either to change out the grips (being a confessed "grip junkie," I have seven sets at last count), or just to see if any "dreck" has gotten in there since the last cleaning. Occasionally, and surprisingly, "dreck" is found.