FCPX video missing a slide
I have MacOs version 11.6 and FCPX 10.6.3. I've just copied onto my MBP a saved project from a hard drive ((from earlier this year). It needed to be updated when opening in FCPX. I had an error in some (pdf) slides in the video that I corrected. However, when I export the video, I get the message: '(Project name) has missing or offline titles, effects, generators, or media. If you continue, these items will be replaced with missing-media graphics.' I continue, and the exported video is missing the first slide, even though it is showing as expected in FCPX. Any suggestions as to how to remedy this please?
MacBook Pro 16″, macOS 11.6