Just to be clear, a United agent has no idea why you do or do not get PreCheck. They can guess, but they really don't know--United sends the reservation to the TSA, and the TSA either sends back PreCheck or not without any additional detail. The best thing a United rep can do is sit with you and double-check that your number is on the record and that someone didn't fat-finger your date of birth.

I retract my earlier comment about asking United the next time you don't get PreCheck on your boarding pass. Instead, ask the TSA -- they are the only ones that can actually know, and the AskTSA Twitter account actively investigates these situations and can help you resolve them.