what he/she said, but

M is an expensive coach fare class, so it "should" be available on most flights, even close to departure and with minimum stay. However, I've noticed that the Shanghai flights seem full (early), especially the new nonstop DTW-PVG service. If you've only checked a few dates or if your destination is PVG (World Expo through October, IIRC), this could be the problem.

Also, sometimes B fares are not much more than M, IME a couple hundred dollars or about 10%. In the days of PMNW, this was handy because NW upgrades from full B came from a different bucket and cost fewer miles. You can combine one way in M with one way in B or Y too.

Finally, have you looked for a good discounted BE fare? IME this works better for Europe than Asia. On my last TPAC trip, M was just under $4K while the best discount BE RT I could find was over $7K about three months in advance of my departure.