Cannot copy opacity masks

I used a template image to create a mask for a video, but once I finished it I realized I could not copy it. Any attempts to paste it resulted in a mask that's the same size as the source. No matter how I copied the mask, it never pasted the shape it originally was. I tried pasting the entire opacity effect, and everything but the mask shape pasted. I tried adding a keyframe to the mask path and pasting it on the other video, and it didn't work. It's not pasting nothing, either. If I paste the keyframe onto a video that already has a mask, it completely gets rid of the mask. What am I doing wrong?