With washes you have to remember to use the opposing paint type.
If your model is finished with acrylic paints, then use enamel or oil based washes.
And vice versa.
Remember to make your wash up so it resembles dirty thinners.
It's easier to add more colour, then to remove, so dont over do it.
If your wash isn't flowing, then it is either to thick, or the surface tension of the wash is to great.