Make sure that the pin on the trigger group that slides into the frame is in the right position when installing.
Hold the slide release against the trigger group and insert the front of the trigger group into the frame and push down. If it doesn't go right into the frame, but stops a quarter inch or so, you will need to get something long and skinny to fit into the space and push over the shell latches into the frame; one at a time will do, while applying slight downward pressure..
That's the only real complication I find with removing the trigger group from these guns.