I had that issue in my recent purchase too Davlaw, I hooked up a high power nozzle on my hose, filled up the sinks in the galley, the head and the aft shower and then put it in the "jet" mode and shot it in the drains! Within seconds, the water just formed into a little whirlpool as whatever debris was in them, seemed to have been blown out with the little higher pressure side of the hose nozzle... They continue to drain well since then.
Good luck!
Ken