I have the same problem. Any note that contains more than plain text (i.e. a PDF or picture) does not display. Instead there is a little note in the bottom right corner stating "Saving". This applies to both the web and the Mac app. The summary view of the note shows some content, which indicates it is still stored somewhere, but there appears to be no way to access this. The help page for 'Missing Content' does not provide a solution for more than one note at a time - and I have more than 4,000...