Heat mat issues!!!

[emwilly90]

I have just set up my tank for my corn snake I'm collecting on Friday.
I have turned on the heat mat, plugged it into a thermostat at 30 degrees Celsius and put the thermostat probe next to the heat mat but it doesn't seem to be getting the vivarium up to temperature, as the digital thermometer I have in the warm end says 23.8 degrees Celsius.
The thermostat has gone off a couple of times already so I moved probe a little further away and the thermostat has stayed on but the digital thermometer is still saying 23.8!
I don't know what the problem is, I've had it on for about 45 mins now. Does anyone have any idea? I have the heat mat on the wall, it is 6" x 11" 7w in a 2ft viv. Is the heat mat too small to heat up this size viv or do I just need to leave it overnight?

I'm new to all of this and I want it to be perfect for my new snake, It's driving me crazy!