Is it always turning to the same side? Stick a tack in that side's back pocket (point up). This should heal the boat to the other side, thus compensating for your stronger stroke on the other side.
When I started paddling (a whole whoping year ago) I couldn't paddle a plastic rental WITH A RUDDER straight! I felt like I was paddling a banana. As it turns out. It was just me being right handed. One side of your body is naturaly stronger than the other and thus develops more power in a paddle stroke. (Most people can't walk a straight line while blind folded...they favor their stronger side it's the same for paddling just more pronounced in some boats). Unless your hull is obviously bent or twisted, it's just your dominant side over powering the other side.
Anyway, it's all a matter of learning how to balance your stroke. And that takes practice, practice, practice. Not that it's a bad thing. I'll take any excuse to put a boat in the water
Have fun!