It may take up to 3 "Drives" by the computer to run evap tests. and as the above poster typed, some need to have many different things met before performing the test, and makeing a "Drive"
Don't know about other makes, but if it is an older Ford, here is what you need:
1) Long soak (like, 6 hours) to get tank temperature down
2) Start up and idle for several minutes
3) Drive steady for a few minutes while at around 25-50 mph
4) Stop and idle some more
This will usually exercise both the idle and cruise monitor.