I have noticed that some missions you receive are impossible to achieve. This isn't too bad unless you let your monarch assign you some or the list slowly fills up with too many duff ones. Perhaps the mission scripter needs tweaking and refreshing every once in a while so that the missions you refused to accept for 50 years get replaced by something you might want to consider.
Some impossible missions:
Royal marriage to a country of the wrong faith.
Build a shipyard in a province with insufficient income.
Vasselise a country that can never meet the vassal criteria.
Some impossible missions:
Royal marriage to a country of the wrong faith.
Build a shipyard in a province with insufficient income.
Vasselise a country that can never meet the vassal criteria.