Check to make sure the path from the entrance is not blocked somehow. One time I made a 2-square wide path going straight from the entrance to an exhibit, with another 2-square wide path crossing the first one. Then I put a fountain right in the crossroads. There looked to be plenty of space around it, but I got ZERO guests. The minute I removed the fountain, the guests started to be generated like normal.
Check your fences. One time I used only downloaded (including official) fences in my zoo, and got no guests. Adding an exhibit that used an in-game fence, the guests began showing up.