Whenever I try to mask a clip or multiple clips in my main composition, the mask has no effect, neither does inverting the mask or any of the other mask modes provided. The same thing happens when I pre-compose a clip from my main composition. Masks do still work on adjustment layers though.
However if i take the same clip from my clips in the project section and create a new composition with it, masking seems to work fine.
I don't know if I have disabled a setting in my main composition that would have caused this (I can't find any difference between my main and any new compositions), and it seems that nobody else has had this problem or that the answer is simplistic.
Let me know if you need anymore information. Any help will be greatly appreciated.