I found it very difficult to measure the temp accurately under the HP as it doesn't work like a lamp and radiate heat very far....
......they put their bodies right against it, like they would a broody mama.
That's why I like a flat rack rather than an arched one, more contact area, especially the first week.
Try putting the thermometer between the pad and rack, or raise the thermometer on a block of wood to get it closer to the pad, and see what you get.
It can be a leap of faith using this method......going by chick behavior more than the thermometer.