Slight corrosion in the caliper cylinders can cause such issues as well. Little used cars and their light weight means pads wear very slowly. I've sometimes exercised them by removing pads and pushing back each piston a few mm with a soft pry, then inserting hardwood strips a couple of mm thinner than pads and pumping brake pedal gently until it's hard. Then removing strips, prying out pistons as before and re-inserting pads. A good time to do this is during bi-annual fluid change.
