Heating problem HELP!

[nazanova]

Hey guys I'm in real trouble right now. I'm due to get a Corn Snake tomorrow but today I've been having trouble getting my temperatures right! At the moment I have a 12W heat mat that is connected to a Thermostat that is set to the highest temp possible but my thermometer on the hot side is still reading 20C when it should obviously be much higher then that. I've tried unplugging the heat mat from the thermostat and running it solo but it's still saying it's 20-22C! Everything is brand new, all windows are closed so I don't know what the problem is? It could either be my thermometer is messed up, the heat mat isn't powerful enough or there's something wrong with the thermostat because I have noticed that on the thermostat there are 2 lights, one that shows that it's on and the other to show that the heat mat is on. The power light is on all the time obviously but I've sat and watched as the light to show that there's heat is on for about 5 mins then the thermostat turns it off! I thought I'd ask for you guys advice before i go and purchase anything. What do you suggest? I'm thinking more powerful heat mat?