Only Y/B fares get a bonus. If it was an M fare, than it was not Y or B.

Perhaps it was an M fare because Y/B was more expensive, therefore not allowing you to spend the same amount and it be Y/B.

It may suck, but just because it was expensive, did not make it the most expensive, nor expensive enough to qualify for the actual bucked you desired. At time of ticketing did you look to see what you were purchasing?