Although there are many investment strategies that are profitable over time, no investment strategy works all the time. Just ask any growth investor from 2000 to 2008, or any value investor from 2008 to 2021. Momentum is no different. Although it is one of the best investment strategies over time, it doesn't work all the time. The difference with momentum though is that we can tell you with fairly high certainty when it won't work.
There are essentially four phases to a full market cycle: bull market, distribution/top, bear market. . .