I saw many topics about it in the www’s but without any solution.
While I am playing solo all is fine but while playing in the public game mode with more than 2 players Momentum is not stacking or stacking a little for example to 11 and then is not going up at all. (neither with SHIFT nor with a normal attack with primary skill.
This is happening from the very beginning with G.O.D set release.
It’s not the public groups causing this, it’s because you’re using Calamity.
Use a different weapon and Momentum stacks will work just fine.