I still make mistakes...every damned day.

I'm hope this issue has been resolved. Here is a hint. I don't know if that troublesome recoil motor was tested and found to be ok??...but if the motor is running and not recoiling it should be a gear reduction issue. I had a JP I built a few years ago that had a intermittent recoil issue. I've built 5 of them and only one had this odd recoil problem. It only worked 25% of the time. I soon discovered it work 100% of the time when the upper hull was upside down. I have not built a IS/JS kit yet. On the JP the recoil unit is mounted upside down unlike most other Tamiya tanks. Is it that way on your JS tank? What I discovered was when the recoil unit was upside down (in it's position of normal function) the gears did not mesh properly. I fabricated and installed a thin clear plastic shim that I made under the large recoil gear to raise the large main disc to fully engage the gear reductions. I used a small rubber "O" ring to shim up the brass center screw a mm or two. When I build a kit I secure all my recoil center screws with Loctite. Or they will come loose over time and drop the retaining nut into the tank (that's why you should cover your gearboxes men). A drop of CA or tape can also be used to keep that nut in place.

I hope this helps. Please let us know how you make out, Bob.