Check out how the servos are installed. One side might have a slot slightly higher than the other side. You'll have to readjust one servo for it to work right. Here's some specific advice I received to set up my F-18:

if so did you remember to offset the servo direction you know one went one way the other went the other way so if you notice the holes for the servos are offset in relation to the bottom of the fus so you can have the servo arms pointed in different directions one up one down then they should move togeather hope this healps