Heating cable

I have made a new propagating bench which is sitting on top of a layer of insulation on top of a slatted bench, the box is made with 1 inch thick timber, internally it is lined with two layers of polythene and filled with a 2 in. layer of sand and a heating cable placed over it this was then covered by a further 2-3 in. layer of sand the sand was then sprayed with a watering can of boiled water and the cable switched on through a thermostate the temperature went to 17.5c then started to lower so I then plugged the cable directly into the electricity and the temperature held a 16.5c overnight. I have set up propagators several times over the years but have never had this problem, the cable has never been used before but was purchased over 18 months ago so can't go back, but is there anything that I could be missing