I also have the "can't get heels down problem". As for trying to stretch things out, be very careful. I managed to create severe pain in my right ankle simply by standing in the stirrups and allowing the weight to sink down into my heels. My horse was grazing at the time so no movement, bouncing, anything was going on and something still went wrong. One minute all was good, the next I felt a pop, something overflexed down there, and I was in excruciating pain for over 2 weeks. I'm still very guarded about really flexing that heel since it's been tender ever since whenever I try to get that heel down despite the fact that it happened 6 months ago.
Hopefully heels down is just a finishing step of the rest of the lower leg being in the right position, because mine will never me optimum either.