I had a problem with my centerboard on a Morgan 27 once. I found that the longer you let it set in the water the easier it is for various sea creatures to find there way between the centerboard and the sides of the centerboard well housing. It wouldn't go down when I wanted it to and then all of the sudden BANG down it went, if I didn't reel in the cable enough. It's a real pain to put your boat on the hard and still be able to inspect the centerboard in the down position. I had to dig a pit so I could lower it enough to inspect the well.
s/v Blue Bayou