That is just the nature of pine. Very high sap. Even a chainsaw will bind faster and need sharpened more frequently cutting pine. I worked as the Sanderman at a local plywood mill (mostly hardwood) for several years here in Oregon. I could run hardwoods like oak, maple, birch etc. with the sanders set up with 80, 100, 220 grit papers for an entire 8 hour shift (4000+ panels a night). When I set up for pine though, the papers grit would plug up quick. On the first load I would be standing at the sanders blowing compressed air onto the papers trying to clean the build-up off the papers as it accumulated....it was a losing battle.