Might be worth reassembling the gearbox and motion but without the motor and check it runs freely. As
P
phils2um
says, it could be quartering or it could simply be misalignment.
If it runs freely - and I mean the slightest push will get the wheels moving, try it with the motor. It may be that the motor needs tweaking slightly on the mounts so it runs without binding on the gears.
My approach is always to think what might have caused the issue in the first place. Has it been run on carpet, so its picked up fibres and gummed up the mechanism, has someone tried to push it along and damaged the gear train, has it been dropped, causing some damage to the gearbox or twisting the chassis? If you can work out what might have caused the problem, fixing it is usually easier.