Hello, Edward and thanks for your question.
TSA Precheck is a randomized system. Not every passenger who has Precheck will get chosen to participate in expedited screening on every flight. This is done to further enhance security measures. I know it's frustrating, but this is how they work. Please read the following from a TSA website:
"TSA incorporates unpredictable security measures throughout the airport and no passenger is guaranteed expedited screening."
Make sure that every airline you fly, has your KTN and knows you have Precheck, so it can be entered into your profile with those airlines.
I guess it was just 'the luck of the draw' that on this particular flight, you were approved for Precheck on your outbound flight, but not on your return flight. It also may have been an error on the part of the reservations agent who made your arrangements.
I hope you get Precheck on all your upcoming flights!
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