Fusion call for help....newb(ish)

I have created a Fusion comp with a 3D camera track past tif’s.
There are about 8 and each one feeds into a 3D merge.
Each still set-up is the same!
Mask > Still > Brightness > ImagePlane > Merge3D

All but 1 are working fine. For 1 the mask does not work. I have tried everything I can think of and suspect it reveals a hole in my Fusion chops...
Any tips would be gratefully accepted

I found that there was more than 1 with a mask problem and after looking long and hard I found the most significant differences between tiffs that worked and tiffs that didnt was the file size: some were 250meg+ so I saved versions without safety layers got them smaller and the masks worked! NB I didnt change the dimensions just the unshown layers that I generate in case I have to revert.