You are probally just getting an air lock and can fix this by taking the hose off till it's under water or hold it upside down in water till it starts. If you hold the cord while it's in the water an just make sure it's taught you should be able to feel the vibration through the cord to tell if the pump is indeed running while under water.
If this does not help you may have to take the bottom end apart and check the impeller for clogs.
I hope this helps and good luck.

Thank goodness for this page and your answers! I was about to buy a new one because I thought mine was broken. I went back out, unplugged the system, THEN put the pump in the water THEN plugged it in. At first it caused the outlet to pop and I needed to reset it, then I unplugged it, reset the outlet, then plugged it into the other outle (below) and then it worked!!! Weird. I was trying to plug it in when it was out of the water and evidently it doesn't like that.