I've experienced this before, too - "down times" for the staff - I can't recall if it was Silversea or Regent where it happened, but I always chalked it up to the staff knowing it's going to get busy soon so now is our time to "slough off". That doesn't mean that I have ever been pleased with it. I have had to beckon them before (which is rare, I hate doing it) but I try to be as polite as possible.

Sometimes I have gotten the feeling that our nature of smiling and being friendly with them at the start of a meal makes them think "Oh, I don't have to worry about that table as much" which kind of stinks. But it's just not in our nature to be sour to them.