1. Try on alternate heat source. If using power switch to gas and vise versa. Recheck after several hours.
2. Remove the outside access cover and carefully feel the heat coil. It should be hot starting at the bottom and gradually going up. If you locate an abrupt change in coil temp it more than likely has leaked ammonia. Usually the smell of ammonia is very noticeable when you open the refrigerator door when this occurs.
3. You can also use compressed air to clean the chimney. If it's partially blocked by a mud dobber, this will give you the same symptom. I use shop air with an air gun that's specially fitted with a 2ft steel brake line that's slightly bent to slide up the chimney after removing the metal flame cover. I'm thinking about selling these custom made tools on my website.