Okay, lets start from step one. Make sure your swash is set on 1 servo 90* as the manual states. Then, double check all of your endpoints and make sure they are all at 100%. Go into your swash mix and make sure everything is set at 60 to start off with. then go into the dual rate and expo and make sure they are set correctly.
On the servo that's not working, or not working like it should, find out what channel it's in. Go into the monitor setting in the Tx and watch the stick movements. Then just try to track down that issue. It's really hard to do without being there, but we will give it our best try.