I've gotten the full search a few times. There's an algorithm that determines how many Pre slots are available at an airport, and sometimes you just don't get it. It's been about a 5% rate for me so far, mostly on tickets booked last minute and one way, but it has happened other times too.
The trick to all of it is to smile, be nice and polite (wish everyone a good day, don't get frustrated, etc.) and just relax as if you owned the place. It keeps you out of secondary screening most of the time, and just makes everything more pleasant anyway.