If it's not a cabling issue, most probably one of the h-bridges diving motors is defect or damaged. We'll then send you a replacement board for repair. Please proceed then to electronics replacement manual.
4. Quick fix - although works fine for extended use as well
Ok, proper hardware manufacturer shouldn't say this, but ... we're not proper. - Szymon, TR team
The easiest and quickest way to fix your wheels when waiting for new board to come is to short-connect two wheels from the Rover side where faulty wheel is located. The bridges can withstand up to 6A of current so there's no problem with driving the motors together.
Find where the faulty wheel is connected to the big 8pin connector, unscrew the cables and connect them together with the same side wheel.
Ex. left front wheel can be connected with left right using one h-bridge as they always act the same direction.