From lights don't wrk when I hit the switch only my parking lights why is that

Both headlight bulbs have good filaments for either hi or lo? Do you lose your light function and then regain it ? like a circuit breaker cools and resets ? Does actuating between hi and lo seem to immediately affect them? There could be more than one cause and possibility if water entry perhaps. inspect the parts and their connectors. Substitute parts during testing to verify and isolate.