As long as your heels are in line with your hips I wouldn't worry about the deepth of them.
Your heels don't actually need to be down that much. If you push them too far down it can throw you off balance and screw up your position
Try finding a ledge such as a stair or a block and stand on the edge and stretch your heels down as if you had your feet in the stirrups. Hope this helps! :)
Do some calf raises to build the strength and stretch in your calves and try not to ride with jockey stirrups. You are mainly looking for stretch in your calves