make sure that you've not got a N/S or E/W confusion going on.
Also, see if you're near a server line. Sometimes the map coordinates are so near a line that it can only be gotten on one side of the line - and in some cases, you have to have your ship parallel to the line, as the way the servers handles boats perpendicular to the line can keep you too far out from the line to cast in the area. In those cases, sometimes you have to go closer to the line in a rowboat, if you can't get close enough on a ship.