Mask Stops/Disappears On Transition. Please help.

I added a graduated mask throughout my project, but I see whenever I use a transition that the graduated mask briefly disappears therefore creating a flickering effect.

Or is there another way to create a graduated shadow on the left and right side of my video? You are my last hope otherwise I'll just create a graduated black-to-transparent image in photoshop and add it above my video.

Thanks for your time!

Yep, that happens. Place the clip and mask into a Compound, then apply the transition to that, sometimes it helps. OR, keyframe the Opacity of the Compound if not.