What do you agitate, Ilford style,
"Fit the sealing cap and turn the tank upside down four times during the first 10 seconds and again for 10 seconds (that is, four inversions) at the start of every further minute to agitate the developer. Each time you invert the tank tap it on the bench to dislodge any air bubbles which may have formed on the film."

I thought HP5+ was 8 mins also in Xtol stock not 8.5 mins, Massive Dev has 8.5 mins, could that extra 30 seconds (6% more dev than the manufacturer recommends) be your issue?