Try "Eliminate", it's in liquid form. It's available in most hardware stores. But be warned, its highly toxic and corrosive, and very very dangerous if you dont handle it well.
Best to put on some long gloves and goggles for this..........
Make sure the area is dry (put the liquid in a couple of hours AFTER you have used the sink/bath) before you use it. Pour it directly into the drain, stand back (it will splash a little) and wait for a while (10 seconds usually). Pour in some water after that to get rid of the smell and your drain should flow without any blockages this time.
Hope it helps...
