There are several reasons birds might not use your bird bath. First, the water in your bird bath might be too deep - as mentioned before, the ideal depth of a bird bath should be 1-2 inches deep. Second, your bird bath might be too colorful.
Birds will avoid bright colored bird baths in case it attracts predators. Instead, opt for bird baths with more natural earthy tones. Also, your bird bath might be too slippery. For a bird to drink from your birdbath, it needs to get a good grip with its feet.
Bird baths require a bit of cover. If your bird bath is out in the open, your feathered friends will be too wary of predators to use it.