First off, make sure that the battery is fully charged and in good shape. does it check out on a voltmeter or expanded scale voltmeter with a load? If the battery is okay, swap one of the other servos with the rudder servo (just interchange the places they plug into the reciever). Now, if the same servo still doesn't work when plugged into the new channel, the servo is bad. If the second servo (now plugged into where the rudder servo was plugged into) now acts up, it is the decoder in the receiver or the transmitter pulse train. If so, you need to send in the Xmtr and reciever to a radio repair service. If the servo is bad, just replace it with a new one.