I am guessing that the servos are in fact centreing when you turn the radio on but the servo horns are not in the right place.

This is only a guess as I don't know any info about your plane. But if I am right. Unscrew the little screw that holds the servo arm on. Pull the servo arm off. turn on the radio and receiver. put the horn back on. Put the screw back on and see if this helps.

Obviously this has made several assumptions. If this is not the problem, let us know if the radio is installed in a model or not yet.

Hope this helps.