If our support team has arranged a Rover Fix for your kit, this is what it is: a small add-on board carrying capacitors that stabilises the voltage reaching the servos. It helps where servos hum or twitch without turning, or cannot find their zero position. It goes onto a finished rover just as easily as a half-built one.
First check: is it actually installed? The board arrives as its own small part in the packet. If it is still in its bag, it is not doing anything yet — fitting it is what makes the difference.
Fitting it. It solders in between the main board and the servo connector. It is a small job, and if you are unsure at any point, reply on the same conversation where the part was arranged and we will walk you through it step by step.
If nobody has mentioned a Rover Fix to you and your servos hum or twitch on fresh alkaline cells, start with My Mars Rover's servos or motors don't move — and if the checks there do not solve it, ask us about the board when you write in. Where it is the answer, we send it free of charge.