We had one encounter where this couple had supposedly been in the lifestyle for years, however this guy was about as lazy, unskilled and unenthusiastic as you could imagine. He talked a good game (hell, his wife made him sound like a modern day Casanova), but was brutally lousy. It was one time I felt sorry for my wife! LOL Needless to say, I doubt this couple gets much "repeat business" if this was his best work.
But, this is the only instance I can recall of someone being a poor sexual partner. Everyone else has been at least eager to learn if not already capable of the task at hand!