Sometimes they never do. Some males think they should spawn WITHOUT a nest. Some take forever. Some build one right now. Really depends on the male.
Helps if it is storming out, though. They notice the change in barometic pressure, and that, in the wild, stimulates spawning.
Of course, if you don't HAVE a storm coming in your near future, then that won't be of help. Other tricks include adding cool water to the spawning tank, unplugging the heater over night and then plugging it back in in the morning (to simulate spring, where it's heating up, I guess). Also, adding spawning aids such as Blackwater Extract, or even something as simple as a styrofoam cup or some plants for the male to build his nest in.