I use the dichromate method for contrast control in my pl/pt prints. I think the main problem you are having is that you are deciding on the proper exposure based on the highlights. This method works for silver paper, but not for pt. Set the exposure based on the shadows. Once you have good shadows, then adjust the highlights by changing the amount of dichromate in the developer. Once the shadows are set, the highlights will fall into place pretty quickly with minimal change to the exposure time.
This is the opposite of the way most people print with silver paper. This is because the dichromate acts as a restrainer on the highlights. As you add more dichromate, the highlights get help back in their development. Dick Arentz discusses this is his book on pt printing and recommends setting the shadows first. Sorry, I don't have a copy of the book here, so I can't give you a reference to the page.