The brand in my description was Sweeter Heater, yes. But it does explain why a traditional thermometer won't work on an infrared radiant heater. I'm assuming OPs radiant heater works the same as Sweeter Heater brands.Thanks Cryss!
The OP is not using a Sweeter Heater which is designed to be hung from a ceiling 6-20 inches above the animal - chicks would not press their backs against that like they would the brooder plate that the OP is using (she's using a something like an EcoGlow brooder plate).
You should still be able to measure the temperature under there. I mean if there is some type of "heat" then there should be a difference from under the plate and just in the tub.
Either way, hopefully the OP will figure some of this out and have no more losses.
"This is an infrared radiant heater, intended to heat the animal underneath. Unless measuring heat with a radiant heat gun, a thermometer will only measure the heat that is released from the animal. It's not a space heater and will not feel hot like one."
Here's hoping she figures this all out. Maybe experiment with a cardboard box brooder?