Why won't my new 'visitherm stealth' heater work?

i got a 200 watt : http://store.petside.com/photos/product/standard/114420S505114/aquarium-gifts/visi-therm-stealth-submersible-heater.jpg for my 50 gallon turtle tank. i set it to 78F. the water was 70F. it has been around 26 hours, and the tank is only 72F. the temp will not increase past that!! what is wrong... show more i got a 200 watt : http://store.petside.com/photos/product/... for my 50 gallon turtle tank. i set it to 78F. the water was 70F. it has been around 26 hours, and the tank is only 72F. the temp will not increase past that!! what is wrong with this heater, people say its so darn reliable, but it doesn't seem so. and i have it right by the filter intake pipe for circulation.

why is it not working as well as it says it does??
Update: thanks Nate! btw i have a digital thermometer with a probe.
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