What do I have to do to be able to recruit other captains? At the beginning of the game they agreed easily. Now they are not interested. Also everytime I enter the tavern I get molested by one of those captains with an offer of attacking a convoy or such :/ I always say "no" because I'm not that kind of guy
I think it's related to your reputation, title, level or ship rank. As game time passes, these captains gain levels and get better ships. If you have a superior ship or fleet, or higher level than them the chances are higher. Let them gain levels and when you want one of them to join you, save before entering tavern.
Don't bother with convoy attacks, I did it once and i found it too time consuming and hard to make it in time, for a normal money reward.