Does anyone know how to complete the “Sunken Empires” scenario?

Does anyone know how to complete the "Sunken Empires" scenario (Heroes IV, Escalibur)? I've been playing for 16 in-game months, but I still can't get to the "Sylvan" faction, even though I've visited all the "Sylvan" outposts! Has anyone played this scenario? If so, could you give me some tips on how to complete it? I'm starting to wonder if it's a bug...