I think that the lack of light may have something to do with the problem if the chicks are away from heat pad when it gets dark or move away in the dark they may not be able to see well enough to find their way back to heat I'm sorry to hear of your loss and hope you figure out problem. I brooded in my basement for years then moved to a out building only to have a heat lamp cause a fire in the building not sure if a intruding animal or human error caused the lamp to get dislodged but it did and $7000 + in damages later I brood in plywood box away from any other buildings. I keep a well secured heat lamp in it and have seen temps in single digits with no trouble. A small light over the heat source may be all that is needed to draw them to the heat if they move away. Hope this helps.
Thank you Shawn, that makes a lot of sense. I never thought of that. I"m sorry to hear about your fire. That must have been terrifying.