Arduino robot not moving straight

I just got an Arduino Robot (the official product) and tried running a simple sketch which makes it move forward and backward. When I used Robot.motorsWrite(255, 255); it didn't move straight but curved. I even tried the logo example, but it still curves. Anybody know how to fix this? Thanks

Turn the TRIM pot. The software uses the TRIM pot to adjust the values you send to motors.Write().

It works perfectly thanks