The probe of the thermostat should be placed directly on the substrate as you're more concerned with surface temperatures than air temperatures. Are you relying on the thermostat to tell you the temperatures? If you are get a digital thermometer, the thermostat dial is useless.
I would also have a search on here about calci-sand - sand itself isn't necessarily bad but calci-sand is not great (they eat it to supplement their calcium intake and can cause internal problems). Do you have a terrarium by any chance (lift up lid one)?