Sometimes even though a fare class is showing via KVS or EF, it doesn't mean that NW will sell it. I can see "Z" fares for this week, but that doesn't mean they'll sell it.

A few things you can do is to search by segment (DTW-NRT-BKK), instead of just the departure and arrival cities.

You can select, search by fare code (V or higher, B or higher, etc), but this will probably still just show your same "M" fare basis.