Hello, Melvin and welcome!
Even though you have enrolled in the TSA Precheck program, there is no guarantee that you will get it on every flight. The TSA does this to enhance security measures. They randomly choose who will and will not be selected to get TSA Precheck on their ticket/boarding passes.
I know this is frustrating, but that's just how they designed the system, to try to make the security measures the best they can be.
Please don't 'kill the messenger' because this is not the answer you wanted to hear. I'm only passing along the information that answers your question.
Whenever you make your reservations, make sure to give the airline your KTN (Known Traveler Number), so it can be added to your ticket/boarding pass and this might increase the chances of you getting TSA Precheck on that flight.
I hope you found this answer helpful and will please take a moment to select a positive rating from the stars at the top of your page, then click 'submit;' this is the only way I will receive credit for assisting you. Thank you very much!
Even after rating, I will always be available if you need additional information; just click 'reply' and I will be glad to continue.