TSA PreCheck is not guaranteed on every flight even if you have your KTN in the reservation. So it's likely that you just didn't get it this trip. But, you can remove your KTN and re-add it to the reservation and see if that helps. That forces UA to do another "check" with TSA. You may or may not get PreCheck at that point.. But I would suggest you be prepared for normal security screening this trip.

I'm assuming you were provided actual BPs and not "see agent at airport"? If the latter then you possibly got SSSS'd which means no PreCheck.

-RM