Steps to reproduce:

1. Attach some PPT and PDF files to a Confluence page
2. Embed then into the page using viewfile macro
3. Result: All of the files were rendered just fine
4. BUT randomly and intermittently, when you revisit the same page again, one of the viewfile macro from all of the viewfile macros will not be rendered - at times only certain page in the PDF is rendered although all of the pages can be rendered just fine previously. The same scenario could happened to the PPT file too. You'll see this kind of error in the logs:

   WARN [http-8534-4] [com.benryan.conversion.PPTSlideServlet] doGet Conversion task timed out for attachment : PK Presentation 2013-11-20.ppt with page number 3, waited : 300, now executing in background.
   

Refreshing the page (at times this has to be done twice) helps to render the affected PDF/PPT file once more. Viewing the individual PDF/PPT using viewfile macro in an individual page do not have the same problem.

Workaround

To force Confluence generate the slide images again:

1. Get the attachment ID for the attachment with the problem. To do so, please run the following SQL query in your Confluence database:

   select attachmentid from attachments where prevver is null and title = 'filename.ext';

   Please replace filename.ext with the filename with the problem.

2. Shutdown Confluence.
3. Go to the folder <CONFLUENCE_DATA>/viewfile/.
4. Remove all files containing the attachment id from step #1. Filenames will look like slide-1234-1 where 1234 is the attachment id.
5. Start Confluence.
6. Go to the page containing the attachment embedded in it. This should recreate the images for the slides.