Re: Working with material on existing projects

It's probably because they're unrendered. When you copy and paste into a new project, you will need to re-render those parts. You can tell what is unrendered by the orange bars above your footage in the timeline.

What about turning the other projects into compound clips, blading, then copy-pasting those pieces into your new project? If you don't want to mess up the original projects, just duplicate them (Duplicate Project Only) then perform this trick on the duplicated versions. Then do a "Break Apart Clip Items" on the compound clips you copied over.