My 42114: 6x6 Volvo Articulated Hauler drives in first gear for a moment, then stops completely; same thing with 2nd and 3rd gear. It doesn’t operate the bucket either. I suspect I forgot to add a pin to the L motor and the orange gear shifter - how can I fix this the easiest way possible? I’ve calibrated about a hundred times already: sometimes it works for a few seconds then stops completely.
It might be the case that the batteries can only supply the required power to move such a heavy model in the first few seconds, then their voltage drops below the required limit for the motors to keep on running. Can you try or have you tried using new, non-rechargeable* batteries?
Adding a missing pin might be trivial in one case and might require a complete teardown and rebuild in the other case. Without knowing which pin did you forget (if any) it is impossible to tell which one applies to your situation.
*: the non-rechargeable ones have 1.5V as nominal voltage, whereas the rechargeable ones commonly top out at 1.2V, which then drops further as they get empty.