I am a routine traveler, traveling on average nearly every other week. Up until early this year, 9.9 out of 10 times I got my boarding pass I recieved my TSA Pre Check. Early this year there was some kind of software change at Southwest, since then, it is rare to recieve. I have checked and double checked to confirm my Known Traveler ID is in my profile, and it is. I have written several times, and get the same response as most folks, it's not garunteed, it's TSA's fault and check your known traveler ID in your profile. These are the standard answers that all having this issue (I see there are many with the same problem) recieve. Would be really noce if someone at Southwest would actually dig into this issue and resolve.
Reach out via Twitter, and have them verify that your name in your Rapid Rewards account is exactly the same as shown on your KTN. And have them verify your KTN is correctly entered in your account. Then, have them verify that your KTN populated to any open flight reservations.