Pre-Check is not guaranteed for every flight - most likely, you were simply not chosen for the program on this particular flight.

I retained my PreCheck after losing EXP, but do not receive it 100% of the time. It's annoying, but they suspect that some randomness can prevent a terrorist from gaming the program. I find that foolish, but whatever.

I've also noticed that last minute changes or bookings can result in AA not passing along the information in time or correctly for PreCheck to be granted.

So what you experienced is not uncommon.