A mask doesn’t show up on a timeline

Hey guys (and everyone), I really need help in understanding why a mask that I created doesn’t show up on a timeline, although I can see it among the layers and in the inspector? 

Maybe it’ll help to know that I created the mask in a new group before adding any elements to that group. And now although I can see that the mask is there it doesn’t function at all. When I add some content to the group the mask doesn’t effect it. The mask also doesn’t show up on the timeline. If I turn the mask into a shape it stays invisible. Anyone knows why? 

BTW I have another mask in another group. It functions as it should.