What's happening at my bird bath? NOTHING!

I've wanted a real bird bath for YEARS.  I've made a couple out of clay pot bottoms, letting the hose drip, drip, drip into the container, and birds DO like that.  They also appreciate me leaving the hose on in the front yard, which has NO grass.  I have too many trees out front for any kind of grass...So, when the mountain hardware store had 3 birdbaths a couple of weeks ago, I bought one and brought it home.  Alas, the birds don't use it.  They completely ignore it.  Does anyone have any advice?  It's a plastic one with sloped sides.  My cosmos plants do seem to enjoy it and one blossom rests in it constantly.  The cosmos do attract hummingbirds, so I see them flying out there a lot, but I've never seen a bird use the birdbath...???  I have now removed the branch that I had lying in it, since maybe that was intimidating the birds?  Does anyone have any advice?