Only one setting is not heating up -- the "high" setting. The other settings seem to be working fine. Any suggestions?

I would say your temp selector switch is bad. I would first check the hi themostat for continuity. If it has then I would test the selector switch for voltage in the high setting. If none replace the switch. Also always be sure the vent is clear from dryer to where it terminates at outside of house. This should be part of routine maintenance