The NXT motors have been curving to the right by about 10-20 degrees. We have checked the program, no curves. We have checked the wiring, but it is also plugged in fine.
It could be that one wheel slips more than the other. This happens when the tires get dirty. Also, you can adjust your center of balance so that more weight is on the drive wheels and they will slip less.
You can also use the gyro sensor to compensate for any slippage of the wheels.
Check your motors.
I had the same problem due to one motor being older than the other.
Try to match motors which have a similar age or usage.
If this is not possible, use a gyro sensor.
Also, check the center of gravity. This can cause wheel slippages.