The board could have developed a fault- modern electronics are reliable, but not infallible.
My first instinct would be to check the ouput relays for the forward tank, the most likely failure point. Then I would check the ouput transistors, then I would check the boards PIC chip.
If that all sounds like double dutch to you, and noone in your locale has the electrical/electronics knowledge, then I will take a look it for you if you're willing to send me the board.