The thermal image of the radiator panel is showing an inefficient panel. The blue area means it’s cold and should be hot. The only reason it is cold is that it will be full of sludge, the image should show an even orange over the whole panel.
During a service, we check for cold spots and can report to you if your system becoming inefficient
The slush builds up in panels is removed with a power flush that should be done every 5 years.