The coupling air bell is most likely stripped. The agitator will lift straight off so you can get access to the air bell, but it doesn't come off easily. I was able to get mine off by getting my fingers under the skirt of the agitator and then pushing on the column itself, slightly to the side with the other hand. Be careful not to smack yourself in the mouth or strain your back. Pull and pull hard!
Patrick, November 2005