There are lots of things that can contribute to that. I assume you have checked for a dragging brake.
Make sure the tension on the axle nuts are correct, not to tight or too much play. If not correct, you will be replacing wheel bearings (and races) at some point.
Then there are possible gear alignment issues that could cause it to pull one way. Checks outlined in the service manual would need to be done to verify this.
Hopefully it is not a nose Torque Tube issue (the tube that runs horizontally behind the rudder peddles that the nose strut attaches to), that is a big job if it needs to come out for repair.
Let us know how your troubleshooting goes.