Bed heat fails

I have a Sovol SV03 (souped-up Ender 3 clone) that worked well for a year or so. It has what looks like a standard Creality-type controller board.

Lately, I can't finish a print because the bed heater stops working either before the print starts or soon after the first layer starts printing. The LCD shows that the controller has commanded the bed heater on, but the temperature doesn't go up. The sensor is giving an accurate report of the bed temperature.

When this happens, the thermal runaway protection shuts the printer down.

I checked for loose wiring and everything looks okay at the board end and under the build plate.

Any ideas on what I can look at next?