When you adjust the weight in the boat you should notice that the boat is easier to row. It should sit right on the rocker (center). When drifting along, it should only take a couple of pulls to stop the boat, breaking the surface tension of the water. Then it should take little effort, i.e., short stokes, to stay in place. Hard to row is hard to hold.

Don't try to anchor in water that is too fast for your set-up. Move to the side, find a seam, or anchor farther up stream away from the fast water.

I'll bet a combination of weight distribution and anchor line scope will cure your problems. If she still won't hold, add some chain to your anchor or between your anchor and rope (I know, it won't go through the pulleys worth a dam).

A stuck anchor in fast water can be a real bummer. Be careful what you ask your boat to do.

Good luck.