TK's inventory control is extremely complicated - they control inventory based on connecting flight ("married segments"), and they also control inventory on the return direction of a flight based on the outbound. They also have some very complex point of sale restrictions which I find it difficult to keep track of. It's very possible that there are still seats available for sale on the flights you want, but not with your preferred flight combinations. Whether it's intentional or a bug is a different story, but you're not going to be able to contact the inventory analyst who can fix it if there is indeed an error causing the issue. Bottom line, ExpertFlyer is not so useful for TK inventory.