Exchanged 2 week old water heater with another new one but still no hot water

Was this properly hooked up to the electric and the electric breaker turned on at the main panel? Did you turn off the electric until the unit filled up with water completely? This is generally the main reason new hot water heaters do not work. The element burned out as there was no water in the tank to cover the element.

Don't know if they turned the power back on the tank before filling it up

Electric I assume, right? Power should be left off until the tank is filled. Who is the they?

I like twinkies thinking here. Who did the install and what steps did they skip because 2 bad heaters in a row is like a 1:1000000 shot.

The Twink nailed it. Two bad heaters in a row? What are the odds?
You either have a problem with the electrical hook up not sending power to the heater **OR** You turned the power on before filling the tank and you burned out the elements. I would first check the power to the heater and if you have power the next thing I would check is the elements. Good luck, Tom