Yes it probably could. Did you let the heater float in the water for a while before you plugged it in? I'm not sure if that's necessary, but the instructions usually say to do that. When I refilled last night my water was very very cold (fishless cycle, don't worry lol), the temp plummeted from 84 to below 70. The heater took a good hour or two to get the temp back up, so starting from scratch I guess it could take that long to heat up initially. If you can get an in-tank mercury thermometer to stick on the glass just to make sure the heater is working properly, it might be a good idea. I keep a thermometer just so I know the actual temp of the water matches what I set the heater to.