I've a flight from A to C (booked as a direct flight which makes an intermediate stop in B). United shows the bulkhead seat as available on A-B, and also on B-C. However, United shows the seat as not available on A-C (which is what my ticket is booked as).

Any suggestions on why this is, and if there is some way for me to book the seat from A-C? The helpdesk simply say that the seat is not available.