If you have the pad right on your heel, then you may feel it more in your calves. Have the pad above the heel on the lower part of your calf (achilles).
Also try them with more plantar-flexion (toes pointing towards ground), makes them harder but takes the calves out of it a bit. If you do lying leg curls try this and you won't be able to move as much weight, but it'll hit your hams harder.