Ozark,
It could be the radius on you rails or it could be another problem. I usually tilt my band saw table to the same angle I ripped my box at and then cut very close to the radius line that I've drawn on the sides of the box. This will help you get them close. Then you can use a belt sander to sand the to the line (or just barely sand away the line). Check pediodically that your progressing evenly on both sides. It could however just as easily be the radius on your lid or the thickness of your walls. If one side of the box is thicker than the other it may not play right. Also if your radius is not consistent on both sides of your lid/paddle then it may not play right. Hope this helps.
Josh
