Never looked into a heated dog bed before. What kind of dogs? Do they sleep together or seperate when its cold? Dogs retain their heat on the surface they lay. Different strategies for different dogs. They may not even be cold and may reject the heated bed because its too warm.

You need to look at how they sleep when its cold out. Do they stay balled up the entire time or do they lay on their side after a bit? If they stayed balled up the entire time then they are trying their best to trap the heat. If they ball up a while and then turn over, stretch out, lay flat then they have built up enough het on their bed and are comfortable. You migh try just increasing the thickness of the insulating barrier they sleep on. Maybe try a piece of 1.5 inch styrofoam insulation under their bed. I had a timber wolf/huskie/sledd dog once and he didn't need any of that. He was fine on the concrete with just a blanket (tough sumb!tch) and moved off anything I thought would keep him warmer.

I now have a lab and a pointer and usually I am only concerned with the pointer getting cold because he dosent have any insulating hair. When it gets real cold like this (5 degrees here this morning) I put his neoprene hunting vest on before I go. Both dogs pile into one dog house where I have a huge costco dog bed stuffed in there. They share and generate heat and are fine although happy to get back in the house when we get home.