Questions to ask yourself:

How old is it? Plants less than two months old are reluctant to bloom.
How much shade is it getting? It blooms best with at least 4 hours of direct sun.
What kind and amount of food is it getting? Not critical, unless it is in very poor soil, or a container.
Am I looking at it at the right time of day? As the name implies, the blooms don't open until four o'clock, sometimes later in very hot weather.
What has the weather been like. Very cold, rainy weather will prevent flowerbud formation.

Hope this helps! To discuss it further, come join GoY: it's free, and we're all friendly folk!