Either the thermostat is indicating the room is already at the desired temperature or the unit is overheating and the thermal protector has shut it down. If this is a new installation check the mounting. If the housing is set too far back into the wall, the warm air may just circulate behind the grille instead of being discharged into the room. The same thing can occur if the unit is installed into a corner where most of the heat is directed right back into the heater. If this is an older installation and the problem has just developed, turn off the power and attempt to clean the unit with a vacuum cleaner. Visually inspect the unit for any restriction of airflow and any damage to the heating element, motor or thermostat.