UTH not heating? Baby in jeopardy?!

[ae88weaver]

I brought my baby home today and set up his cage. I'm using an 8 gallon rubbermaid and attached the UTH (Zoo Med, small size, 8 watts) and judging by the 2 thermometers (one digital, one normal), the heater is not heating right... I attached the UTH underneath the outside of the cage, covering only one side, about 1/4 of the cage... I can't get the cage above 80 and that's pushing it and the UTH has been on there for about 3 hours. Although the size I chose said it was for tanks 10-20 gallons, did I make a mistake? Is this thing defective? Or are both my thermometers off (although they read my air temp correctly)? Should I use an alternative heat source? Should I get a larger pad? HELP! I don't want to put my baby in jeopardy!