Help Me! Can't get my Top Off to Work!

So I just got a Tunze Osmolator nano 3152 to top off my tank with rodi water.  Read the manual, followed the instructions,set it up accordingly and it doesn't pump water.  Yes it is plugged in and there is power, every couple of minutes it "buzzes" like it is trying to pump water.  Question-my rodi tank is about 3 feet below my main tank.  Do I need to move the rodi tank higher?  Does this pump have a short distance it can only work at? 

Does anyone have this model, or have any suggestions?

Thanks