The result of developing unexposed Harman DPP is black. If you get white the paper is grossly over-exposed or the developer is dead. Harman DPP needs strong developer to give a decent black. I use fresh warm Dektol 1+2.
Pre-flash amount is critical, +/- 10%. I give 0.8 seconds at f16 under my 4x5 enlarger with the (empty) negative carrier 900mm above the base board. Your calibration will differ. I always pre-flash with the Harman DPP in a film holder. The edge rebate delivers a reference maximum black to compare to the other tones.
Ordinary safelights will disasterously fog Harman DPP and deliver white frames in the developer. I use a very dim deep red safelight.