Bittle back "legs" will not respond

I have completed assembly of Bittle, and moved on to calibration using the "Petoi desktop app." However, the back legs do not respond to adjustment during calibration, or move when the robot changes position to "rest, walk, or other commands. The front legs move normally, following commands from the remote and calibration set-up. I have disassembled the robot and switched the motors in the front and back legs, but the results are the same. When the back legs are manually moved into the rest position, they do return to calibration position, but do not respond after that. The back left leg will shift slightly up at the second joint when the robot is told to stand, but that is all. Does anyone know how I can fix this problem? I have attempted to calibrate several times.