Step one: decide to dye an unrealistic amount of fabric
Step two: soda soak it all and wring out batches and put in assorted dishpans with good intentions of getting to it "soon"
Step three: after a couple of weeks when the fabric has completely dried out find some time to dye.
What happens, is that the soda ash forms crusts in the folds, and particularly the selvages. This crust doesn't dissolve even if you rewet the fabric. The crust then acts as a resist, stopping the dye from reaching the fabric. You can see how dark some of the dye bath was, but nothing got through the white areas where the soda ash had dried solid.
|Soda ash makes resist at top right|