Occasionally when I open a project to contiue working on it, all the rendering appears to be required again. All the project files are kept on internal raid drives on our mac editing system so nothing has been moved. It is as if the project has just forgot about the render files all of a sudden. This trouble on long projects where I need to make a quick change and then export using rendered previews as is oftern the case with our projects.
In this case I know the project closed without a problem so I am puzzled.
Issue could be project specific however the rendered previews are stored in the audio & video previews folders.(project settings>scratch disk) check if the locations are available and accessible by the program. Also check if the location of these audio & preview folders are located in the internal hard drive.
If the problem is re-occuring try archiving project(project>project manager), and then open project from the project file within the archived folder, render project and try to recreate the issue.