I’ve had exactly this issue with my Veritas plow plane. I believe it’s due to the sides of the cutter, which are square to the face of the cutter, binding in the groove. I have since ground a slight bevel on the sides of the cutter, being careful however not to change the dimensions of the cutter. I’m not concerned with getting the sides sharp, just beveled enough to provide some relief in the cut. In tough or uncooperating wood, the other thing that helps is Paul’s method of using a marking knife to score the sides before plowing further. This works amazingly well if there’s a knot in your path.