Ferrries are not necessarily daily as you noticed. The Korcula-Dubrovnik bus route is a bus that runs *early*, crosses over to the ferry port for a brief vehicular ferry to the mainland and then goes by road to Dubrovnik. There is a private van service that runs a little later, does pick up and drop offs at Dubrovnik. I learned it is cheaper to book this when you get to Korcula. Their offices is outside city walls, across the road from the path/stairs up to the main gate, and opposite the Konzum.

Be aware that there are at least two ferry terminals in Hvar. One (foot only) in the main town and a vehicular one quite far away from the town (there is a public bus that connects with the ferries).

http://www.korculainfo.com/ferries-korcula.htm is a good place to start your research