Hello, and welcome.
The TSA Precheck system is a randomized one, to add further security measures. Not all passengers enrolled in Precheck will be chosen to receive it for every flight.
If you would like to try to increase your chances of getting TSA Precheck, make sure that you notify every airline you fly, of your KTN (Known Traveler Number), so it can be included in your profile and also mention it when you make your reservations.
I hope this helps.
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