Grey Water Tank is draining really slow now....

Did this just start or has it been a problem all along? Our problem was that they left a cutout in the tank for the hole where the drain tube attaches. Dealer took the pipe off and removed the cutout that was the issue. It would drain ok at first then get down to a trickle as the cutout was sucked down over the hole. We are fortunate enough that our galley has its own tank so it never gets drained unless it is at least 1/2 full or better. With a family of 5 that is usually not hard to do.