It's the combination of Dektol, the paper, and TF-4. I had that problem when I first started using TF-4 for paper.
Be sure to use a full 30 second rinse for RC or 1 min for FB in reasonably fresh water between dev. and TF-4. I fill the water tray as high as is practical and change it periodically during long printing sessions. Also be sure the fixer is not exhausted.