Originally Posted by
FlyDeltaJets87
As noted above, this is common with some 3rd party sites. For example, government reservations are booked through the Defense Travel System, and aren't ticketed until 3 business days before the trip begins. Since the trip is not ticketed, you can't select "Preferred" or "EC" seats online (HINT: you can, for now, at least select "Preferred" seats through the DL Mobile App - EC seats still come up blocked). Once the flight is ticketed, you can select any seat you're entitled to - Preferred or EC.
Interestingly enough, after 10 years of constantly booking travel through the same corp travel agent this is the first time I experience this problem. I usually just go to delta.com as soon as I get booking confirmation email and voila! Some policy change probably took effect recently that introduced this delay...