This is likely because the seat has no price band attached. Follow these steps to check and amend:
1. Go to Events and select your event
2. Select the performance you want to check
3. Scroll down and click 'Edit Capacity'
4. Zoom in on the venue plan to find your seat. If it is greyed out then it has no price band attached:
5. If you'd like to add it to a band, click on the seat and then click the + icon on the band you'd like to add to it
6. Go back to Box Office and check that you are now able to book it - FYI you may need to refresh the page in order for it to update
MORE INFO: Editing capacities on a performance