Ceiling radiant heat
I live in the Pacific Northwest where ceiling cable heat used to be very popular. When I have found heating problems with not getting hot enough where it did heat properly in the past it has always been the thermostat with 1 exception. The 1 exception was a parallel cable set in a large room where a swagger lamp toggle bolt was drilled through 1 of the cables. Other than that my experience has always been bad contacts in the thermostat for partial heat.