This question comes up frequently. There has to also be a fare published between the cities you're searching for that uses that fare bucket, and you have to meet the criteria for that fare (e.g. advance purchase, minimum stay, etc.)

It is extremely common to see an individual flight with fare buckets that you cannot buy because there is no fare based on what you are looking for.

Another thing that can affect this is that the airline may sell fares for a connecting pair of flights, one of which may be the individual segment you're looking at. So there could be an S fare (for example) between BOS-EWR-IAH, but only M fares offered for the same EWR-IAH flight. In this example you'd see S inventory on the EWR-IAH flight but couldn't buy it. There are huge numbers of examples of this type of situation.