Try giving your chick some sugar water- water that's mixed with sugar. Add stuff to their drinking water like some Apple Cider Vinegar with the mother or that other stuff that I'm not quite sure what's it called.
As far as heating, maybe you can try a heat plate? The heat pad might be an issue, though you seem to doubt it is, so it might be fine. I'm just concerned it could be burning them, causing something enteral wrong like the crop, electrical shock, or something else. To test, place your fingers or hand on it for a little while in different spots. Be checking for an electrical shock or anything like that that could be a defect.
Once the heat plate is ruled out, check for anything from pasty butt to swollen crops to inlarged veins. Where did you get these chicks?
@Eggcessive