From the look of the setup I'd say the substrate is probably too deep as a starting point. Anything over 1 inch deep might be insulating the UTH.
Also, is the tank sitting directly on top of the UTH with the weight resting on it? If so, this might have broken the UTH. The tank needs to be raised slightly off the UTH to allow airflow over the surface. Could be it's alerady overheated and gone AWOL.
Could you let us know if you think the UTH and hot rock are both slightly warm or completely cold? As they don't have a thermostat attached, both should be well above 100 degrees (dangerous for Corns and enough to make you take your hand away pretty fast) if they're actually switched on.
At present, it seems quite lucky that they don't appear to be working - Corns are safer too cool than too hot and if all your kit was working full-blast, it'd be very bad for your little guy.
I suspect the shop has done a good job of selling you lots of kit, but not such a good job of selling you the correct kit for a Corn. Sorry, but it happens. We'll do all we can to help. Bear with us and keep posting answers while we try and figure this out.