DIAGNOSIS The warming plate has become warped or deformed from high, continuous heat, or the carafe lid handle is interfering with placement.
FIX First, ensure the carafe is perfectly clean on its bottom and that the warming plate is clean. Any residue creates a gap. Try placing the carafe without the lid to see if the lid handle is hitting the machine body. If it's a plate warp, it's a physical defect.
IF THAT DIDN'T WORK The spring-loaded switch under the warming plate that detects the carafe may be stuck or broken, preventing the warmer from getting full power. This requires internal inspection.
HEADS UP The borosilicate glass is tough but can crack from sudden temperature change. Don't pour cold water into a hot, empty carafe on the warmer.