I have no info for Delta, so I can't help you there.

You might improve your chances by flying mid-week, instead of Sundays. My wife is a United FA and we try to go to the islands a couple times a year. As a general rule of thumb, I don't even bother checking loads for Fridays outbound (from SFO and LAX) and back on Sundays as they are nearly always full. Flying Tuesdays, Weds, or Thurs are much, much better options.

FYI, we almost went to Croatia 2 weeks ago. We ended up in Malaysia and Singapore.