Heater not working

Question
Hi. I had a Tetra submersible heater in my aquarium (which is a 20 gallon one) and it stopped working. The temperature was only at 70 degrees, which is room temperature. I replaced it with an Elite submersible heater, for a 20-35 gallon tank. The heater light comes on periodically, but it didn't heat the tank up over 70 degrees (I had it set for 76 for my tetras). I returned it and got a new one (same brand and size) and that one also didn't work. So today, I purchased a Penn-Plax one for tanks up to 30 gallons and the aquarium is still only at 70 degrees, even though I also set this one to 76. I have no idea what to do. The aquarium has a hood, and I know the thermometer is correct. I'm positioning the heaters away from the filter, and the "on" lights come on here and there. I'm worried the fish are going to die once it starts getting colder. I'd really appreciate any suggestions.

Answer
When you are buying heaters, I would suggest a good brand because many heaters malfunction and it will cause death.  I have heard so many issues about heaters getting stuck and "boiling their fish" or heater stops working during the winter when they are on vacation.  You are buying the appropriate size heater for your tank.  It might be the heater itself that is not working right.  Try the Marineland heaters.  They have very very good reviews.  I also use them myself and have had no problems.
-Matt-