Make sure the heater is sitting between 1⁄8 and 1⁄16 inches from the heater cover. If this doesn’t solve the heating issue, than check that the shape of the heater cover is close to your old heater cover, compare it to our website pictures, or send us a picture to verify there is no defect in the shape that might cause this problem. Our heater covers have always, and will always, be made of borosilicate glass.