Are you using the hanging bottom config with a piece of flex watt in a piece of metal that the tub slides over for your rack or the solid bottom with flex taped down to it? The hanging bottom does not regulate temps well at all as far as my experience. (6 rack tub that needs redone!) If you are using a solid bottom and it still is not working try closing in the rear and sides of the rack to give the heat only one method of escape!