Perhaps the OP is confusing fare bucket inventory visible on EF and fare rules, which would be taken into account using a booking engine or ITA/matrix? EF is going to show you real-time inventory, but that doesn't mean an "available" fare class is bookable. A published fare may require specific dates or direction of travel (outbound/inbound rules may be different), etc., so you may see availability in a "low" fare class, but be unable to book that fare class if you are not able to meet all the fare requirements--so when you try to price the trip you will fall into a higher class of service that allows the dates and directions you requested. You can see fare rules on EF, but finding and understanding them is easier said than done at times. EF is not designed to build a bookable itinerary for you, only to show real-time inventory. My guess is that the fare you are interested in requires outbound travel in one direction, so if you search simple inventory for your "return" segment, you may see availability, but you won't be able to book it as the fare rules are not being met. Hope this helps.