I have been using a Futaba R149DP receiver with JR retract servos(NES 703) on four of my planes with no problem. However, on my next plane a WM P51 GS, the two retract servos do not work with that receiver. I have spent hours looking trying different combinations to identify the problem:
-Two JR retract servo connected to the receiver with a Y harness does not work
-Two JR retract servo connected to a airtronics receiver with Y harness works
-One JR retract servo connected directly to the Futaba receiver does not work
-One 3001 Futaba servo connected to the receiver works
-Use of servo test on my Futaba C9 radio works the servo with some jerky movement
-Connect channel 5 on the receiver to the flaps with Y harness works

I am using 6V batteries, could that cause the problem. Does the JR retract servos(NES 703) work on 6V batteries.

Retract servos are typically servos that run from one end point to the other, and are not proportional. It's very likely that the signal pulse your JR servo is looking for and the one the Futaba transmitter sends are not compatible.

Please remember that each manufacturer of equipment doesn't take other manufacturer's products into account when they make their designs. It would not be considered unusual if equipoment of one brand had compatibility issues with equipment of another brand.

I finally found the reason to be that the JR retract servo does not take 6V batteries. This surprise me intially but when I used Futaba retract servos with 6V batteries it works but not the JR.