Are those servos on a "Y" harness? If so, they may be loading the signal line from the receiver to a level below what the servos need to work. You might try them individually to see if they work. If not on a "Y" harness ..... I'm out of ideas other than buying new servos. Those were good servos in their day though a bit small and delicate for the task on that size of airplane. They've been out of the Futaba product lineup for at least 10 years so they're getting a bit old.