I bought a 12 watt, 11x11 inch habistat heat mat and attached it to my thermostat, but so far it doesn't seem to have gotten any warmer than 20C. Is it just working incorrectly, or is the wattage is too low? I think I need to buy a new one, can anyone suggest a wattage or brand to get? (Or give some suggestions).

(FYI: I have an EVO LITE thermostat and have messed around with the temps, setting it higher than it should be, but so far nothing over 21C.)