I had exactly the same issue. Batteries 100% but re-checked with the supplied battery tester/charger, all batteries were ok. R2 jumps forward, red light comes on but head didn't turn, then LCD displays flash. I noticed that when it jumped forward on start up, it was one sided as if it was about to do a circle, looking closer, I noticed only the left leg had a blue light lit.
I then removed both motor covers on the legs to check they were both plugged in correctly and that the cable colours were the same on each leg, they were. I then removed the rear cover (disconnecting the speaker) and then swapped the motor cables round (L.FM and R.FM - see pictures) on the main board to see if it was a fault with the board, or the right leg motor/wiring. The right leg worked ok when plugged into the left leg socket, so it looked like a faulty board.i then plugged the left leg into the right socket but this also worked, so I plugged everything back where it should be, but noticed that although the plug was fully plugged in, this does not mean that each pin in the plug was pushed fully home. So I took a small pointed pick and individually pushed all of the pins fully home. Powered up and everything then worked and has been now for almost a week.
This is the same for all pins in all of the plugs (see before & after picture examples), it seems that if R2 has registered a fault, then the LCD displays flash to inform you of this, once this happens, he won't continue or at least not correctly dependant on the fault.