Using Strathmore Bristol (Smooth and Plate) and traditional cyanotype formula 1:1, I expose until the darkest parts of the print turns light grey, sometimes refered to as 'reversing'. Then I lay the print face down in a pan of cool water from the tap (no wash aid) and every 5 minutes I change the water without ever allowing the running water to directly hit the surface of the print. After 4 changes of water I let the print dry.
I am no expert, but I have seen where water hitting the surface of the print will remove the emulsion and result in a lighter print, and where overwashing will give the same results. You might try cutting out the washing agent, adding a little vinegar to the wash water, reducing the washing times, or trying some different papers. Whatever you do, do one thing at a time and compare the results. Changing too many variables will not allow you to narrow down the problem(s).