Nothing seems out of the ordinary in your developing procedure on the face of it, so I would consider possible exposure issues.
Are you adjusting the film speed for the developer you are using? If you haven't already done so, go to the Ilford website and download the technical data sheets for these films, and note the recommended speed and developing time for each combination, and note that these are only starting numbers that you may need to adjust for your own system.
Also, be sure you've got good negatives at the normal speed before you try push or pull processing.
Here are two combinations that work for me with Delta 400 Pro (120--35mm may be slightly different)--
EI 200, Perceptol stock, 20 deg C., 12 min, agitate for the first ten seconds then 4 inversions per minute thereafter. Should produce fine grain for a 400-speed film, delicate highlights and rich shadows.
EI 400, D-76 or ID-11 (1+1), 20 deg. C., 14 min., agitate as above. Grainier than above, but still a nice tonality with a little more speed.