Hello,sir. In your HP, is the magazine disconnect (safety) still intact? If so, it may be pressing against the upper front of the magazine and this can keep the magazine from dropping free when the release is pressed. If this is the case and the sear spring is also touching the magazine, the problem is compounded. In most cases, I've found the magazine safety to be the culprit and I routinely remove them on my HPs. Before you do that, be advised that many strongly disagree with this as they have concerns over civil court problems in the event of a defensive shooting and there is merit in their opinions. The only other thing I can think of concerns the use of Pachmayer rubber wrap-around grips. Sometimes, if these are really tightened down, they'll press against the magazine such that it'll not only not drop, but can be manually removed only with considerable effort. Best and good luck.
