Are you checking the rx battery,through a switch harness or right off the battery conectors.I have had a bad switch and when I tested with my tester through the charging jack in the switsh harness the batteries were low ,disconected the battery from the switch and tested it it was full. Bad switch and harness,changed it ,it was fine.This is one reason people like to run two switches and batteries.Might not be your problem but it is worth a look.If not sure Change it anyway,Really what is $10.00 for a main componit in a $500.00 or more plane.

Joe