Delphin 155 Surf: foot brace rails

has anyone had trouble adjusting foot rails on brand new Delphin 155 Surf kayak? I have a used one, which is pretty easy to unscrew and screw on by hand (with the knobs). But my friend got a new kayak and they are so tight, that they just rotate with the screws on the outside. holding the screw from the outside in place and using pliers on the inside with big man muscles didn’t seem to work, without stripping the knobs. So we’re looking for any suggestions on how to get these things off to adjust the foot brace.

they may be galled
The threads may be galled by debris or cross threaded. It would be fairly easy to take the screws and knobs to a hardware store and replace the entire assembly. You may have to up size the screw and knob by one fractional size to make them fit tightly in the hole that is drilled in the kayak, this may mean drilling the rail to accept the larger screw loosely so it will be tight in the hull of the boat and loose in the rail.