Why do people advocate being anything less than perfectly clear about an issue that is bogging down the raid? Do you really want to play the game with someone who is going to flip shit about not making a worthwhile contribution and isn't willing to make an effort to?

Just catch them after raid and tell them about it and that you'd appreciate it if they tried working on it. Because to me it doesn't sound like it's highest on your list of priorities, but that it'd be nice if they could try to contribute just a little bit more.