Unfortunately there is no rhyme or reason to it. In fact there could be two W class fares--one is flex and one is saver. The only way to book it is to have a travel agent specify the full fare basis code when storing the price. If you have access to the fare rules, what I do is start from the lowest fare and keep checking each fare up the ladder to see which one is flex. Then if it can not be booked online as the saver fare is the only thing that comes up, find a travel agent that can issue me the fare that I want. In some markets like ex USA where saver fares are often offered, it has become a big pain. EK/Skywards still don't seem to see it as an issue, so either lots of people are booking saver fares and thus they are paying out a lot less miles and giving fewer people status, or enough people are paying a higher premium than they should to get a flex fare. Either way, it seems there are no plans to make it easy to book a flex fare.