Could be any number of reasons.
- No W class fare defined between your start and end points.
- You don't meet the advance reservation, minimum stay, or maximum stay rules for a W fare
- W fare not available on your return leg (if a roundtrip) and the fare you are using for your return is non-combinable with a W fare, or vice-versa
- You are booking for two or more people and only 1 W fare is offered
- KVS listing is out of date
- KVS listing relates to a different point of sale than that which you are looking to book from
- Married segments
- Starnet blocking
OK maybe not the last one.