9.9 4 stroke 2001 Won't shift

Just bought a 2001 9.9 Mercury. Has the Quicksilver 3000 shift control box. Took the boat out. Everything seemed fine at first. Pushed the fast idle button in to help the motor warm up. At the point I could no longer shift. Won't come out of neutral. Something wrong in the shift box. I've taken it all apart and can't figure out why the shifter won't engage. The black plastic cap for the fast idle was missing 2 of its 4 teeth. I've order a new one. Also, trying to put the thing bag together was an absolute bear. I just gave up. Any tips on this situation? Thanks!