Originally Posted by anabolism

The lower 'D' availability could be because of married segments or a point of sale restriction or a type of fare restriction or a point of origin restriction. You can check for married segment inventory in EF by doing a flight availability query using the connection point. You can use EF's point of sale control to see if the availability is different for different points of sale. The past few years I've seen a lot of 'D' (and 'A') inventory restrictions for xONEx fares. In those cases you should at least be put on the waitlist.