Hi Tom, that does not surprise me. We know there is nothing wrong with the TCB hardware, since it can perform the recoil every time after the first. That leaves firmware or something mechanical with the recoil unit itself. Your second TCB has the same firwmare, and you are testing on the same recoil unit, so the result is the same.
There is nothing I can think of that would account for the behavior you describe, but I have nevertheless taken a stab in the dark at some modification of firmware that may or may not have anything to do with it. At any rate you can test it. Connect your TCB to your computer, read the settings into OP Config, flash the attached hex file from the Firmware tab selecting "Use your own Hex," then when that is done, re-write your settings back to the TCB.
Then you can try and see what happens.