Are they acting like they're cold? Standing all hunched up, huddling really close together, making peeping noises that sound distressed?
I'm not sure what type of heat pad you're using, or how it's set up. If there's only a small gap between the floor and the pad, they might need to be pushed through it a few times until they get the idea. I have to do that sometimes with my heat plate, when it's set very low down for brand new chicks.
If they aren't acting like they're cold, it might be that they're mostly ok huddling together under the lamp and only need extra heat for short periods. I've had a few chicks that would sleep with most of their body sticking out from under the heat plate, or even up on top of it, from a day old at temperatures in the high 60s.