My first questions would be, what is your ambient temperature like? I know when I first started, I had a dedicated room in the basement, but my ambient temps where about 70°F. With heat tape, getting my hotspots was mediocre at best, even with closed racks. I soon realized that heat tape is not a miracle worker. Once I heated the entire room to an ambient near 80, then my heat tape started working more effectively.

If the room cannot be heated and ambients are low. My next step would be to try and insulate the rack, assuming you are using a rack.