When does your heel come up? Knowing what you are doing at the time can help others try to figure out what is going on.
Something to do in the mean time is to stretch your calf a lot, so that it has the elasticity needed to let your heel drop. And don't think about your heel going down, so much as visualising your weight going through it. Shoving your heel down as hard as you can isn't correct (not saying that you are doing this, just making sure that you are not).
Stand on stairs with your heels hanging over, let them drop all the way down and wait two seconds then bring them back up and hold for another two seconds then repeat again until you can no longer do any more. Do that several times a day. :)
Stand in your stirrups, with both long dressage length leathers, and up short in 2 point. Heels come up because the rider is gripping/holding with their knees and thighs, thus lightening the seat. If your horse decided to buck, or tripped, you'd be over the front before you could say 'bugger!'!
Getting up in your stirrups will help you find your centre of balance, if your heels come up you'll fall off, so you learn pretty darn quickly where to keep your lower leg. It's also physically impossible to grip your knee and thigh in this position.
When you return to having your seat in the saddle again, you will have a far more stable seat and your horse will work better for it.