You might have a slight warp in the wing or the vertical stabilizer might be out of alignment a little bit. There are probably numerous other possibilities as well that only a careful visual inspection can really diagnose.

The simple, easy, fast fix is to use the servo linkage to adjust the rudder to the left a little at a time until it flies straight with neutral trim. This probably will not result in the most efficient airframe, but it should mask the problem and make it flyable.