it is possible that you didn't get pre-check because you've recently engaged in some suspcious behavior that has gotten the attention of some governmental agency and/or their algorithms, but this is not what's going on for 99.99% of people who don't get precheck when they think they should have. The idea that the TSA just "randomly" decides not to give it to you sometimes is bunk. There are random elements in screening but this isn't one of them.