What is route? Note that simply having I/Z bucket availability on flights is not sufficient. There must be actual I/Z bookable fares available for purchase on flights. There are a number of reasons why that might not be true. There may be no FC fares applicable for route (as mentioned above). There may be no married segment I/Z availability on flights (you should be checking inventory on EF for entire route, not hop-by-hop as booking availability is based on married inventory, not individual flight bucket availability). While rare, there also may be no inventory in coach buckets corresponding to I/Z fares as domestic FC fares use dual inventory checks and there must at least be some U/T/X/V coach inventory, as well as Z, to book one of the Z fares on route and there must be Q/K/L coach inventory, as well as I, to book one of the I fares. Advance purchase requirements can also be an issue if you booking very near in (although there are usually at least some I fares with no AP requirements).