I agree with previous comments by both Lee and Tom. I think you are under-exposing the film and in the best of curcumstances HP5+ is a poor candidate for push developing. A better film for this would be TMY (Tmax 400), but to hope for a real gain of one stop in effective film speed is probably expecting too much.
Originally Posted by Tom Hoskinson
Finally, semi-stand development will produce maximum effective emulsion speed but don't expect a real increase in effective film speed of much over 1/4 stop. Unfortunately most of the film speed is built into the emulsion and developers and type of development play a relatively minor role in increasing EFS.