
I'm so old that I still use my heat lamps in my brooder setup for the past 30+ years. I have a remote thermometer sensor under the heat light and can read the temp on my base unit in the house. I still think the most important thing for days old chicks are warmth, water, and feed. Probably in that order.
Have you checked the temp under the brooder plate? Just curious to know what the temperature is for your chicks. I still go by the first week set at 95F, the second week at 90F, etc... until the chicks are fully feathered out.
IMHO, one big advantage to using heat lamps is that you can visually see how the chicks are responding to their environment at all times. They are never hidden under a plate, or brooding tent. But I'm just comfortable with the way I learned how to brood chicks 30+ years ago. I have not changed anything.
I hope the rest of your chicks will do fine. Good luck.