We don't know anything meaningful about your workflow. Are you trying to use the DI as a track matte? Did you render an alpha channel? What does your composition look like? Are you simply trying to add a mask?
If, for example, you rendered a black and white high contrast DI and want to use that as a track matte you would place your DI above the layer you wanted to mask then select Luma Track matte from he options on the next layer down. If you rendered a DI with an alpha channel then first verify that the footage does indeed have an alpha channel by either using the Info Panel or using the channel preview options in the Composition Panel, then put it above the footage you intend to mask and choose Alpha Matte or Inverted Alpha from the track matte options. If you do not see the Track Matte options then press the F4 key to reveal them.
AVI DI - Its a bar of soap in a womans hand.
I created a feathered mask on a duplicated top Layer using Pen tool. Mask is centred and focused near centre of soap.
The identical companion layer is at the bottom of the comp.
Objective is to have in between layers of fruit, fly in and disappear / merge into the soap. ( Ingredients)
The mask - effectively 1/2 the screen, has a Black side assumably alpha... but nothing I could do would show the fruit flying at the soap.
Until I tried using the AVI DI in its own COMP that I then nested back to my working Comp.
The DI Comp was set up just as I had with the AVI and the Mask worked perfectly this time.
Just wondered why the source AVI did not work? Duplcated layers is kind of a fundamental masking technique for me and AVIs are the usual export I do for DIs.