Need help with my ZS film/ paper test

I chose to go through the testing process proposed by the late Fred Picker in his book “Zone VI workshop” (plus same on DVD) and got stuck at the point where I’m suppose to find the personal developing time for Zone VIII. I cut the film exposed for Zone VIII with my PEI into 4 pieces and developed three of them based on:
-manufacturer time: 9’ 45”
- less 12.5%
- less 25%
My problem is that all three pieces exposed on paper with the time found for the max black gave me paper white, no trace of gray whatsoever. How is it possible? I’m probably doing something wrong. I can cut developing time even further say minus 35% which would be at 6min 20sec but then I run into uneven development. Any idea from the ZS experts? Here’s what I’m doing:
Film used: Kodak Tri-X – 35mm
Film developer: D-76, dil. 1:1
Paper developer: Dektol, dil 1:1
I determined a film PEI of ISO 160 and used same to shoot the Zone VIII negative I mentioned above. I determined the paper developing time for max black, using F8 on my enlarger and 27 sec exposure so I expose the three Zone VIII negatives for 27 sec to no effect. Thank you

The film pieces are overdeveloped. If you processed them individually as little strips, then that is the problem. If you are doing 35mm, you really need process the Zone VIII images in the tank with a whole roll of pictures. (Hey, it gives you an excuse to go out and shoot a roll!)

Thanks for the suggestion ic-racer. Yes, I can see that as a reason. I will definitely change the way I develop the strips. I will load them into a double roll tank along with a normal roll that I shot with the same camera. I will report back ASAP.

You should still be OK down to 5 minutes or so, if needed. I have had acceptable results at 4 minutes. When I first got my Jobo I spent a lot of time 'chasing my tail' when I was processing little strips of 35mm at at time