Is the tank it's on, full? It needs to be - at least until it gets going (it will keep its prime surprisingly long after a tank is neglected and evaporates...)
Make sure you prime it by filling the chamber with water to get it started.
Make sure the holes in the spray bars are clear - if they're clogged, clean them out with a toothpick or bent paper clip.
In the middle there should be a little gray button. Pull that up to open up the flow as much as possible.
Jenn