There are three components controlling the reverse. Either one of the three could cause the problem. In rare cases the microswitch has corrosion on it and will not engage the reverse contact, but that would be pretty obvious when looking at it.

If you want to rule out motor malfunction, you can reverse the polarity of the motor, by unplugging it from the board, then reconnecting it the other way (black instead of green and green instead of black). If the motor now turns to the other direction it one of the above mentioned components.

We can fix it for you if you want, or if you are handy with a solder i can try to help you DIY.