iTunes loses link to mp4 artwork on My Cloud

I store mp4 movies on a WD Cloud PR4100 and play them using iTunes on a Mac. Normally I can see all attached metadata on iTunes, but every now and again the links to the artwork associated with individual files are lost. This problem seems to occur initially when the connection to the NAS is lost. The links can be re-established for individual files by clicking “Find Info” and then “Return”, but this is not practicable for thousands of files. Does anybody know what might be causing this problem, how it might be averted, and how it can be rectified easily for large numbers of files?


You could refer to the following link:

Many thanks Jonty. However, the link you provided isn’t entirely relevant. It refers to instances where the Library.itl file is stored on the WD Cloud, whereas the problem I am facing occurs when this file remains on my laptop.
For others facing the problem, though, I have found that the problem tends to occur when changes are made to the iTunes library in parallel with a Time Machine backup. The problem can be avoided, therefore, if the backup is postponed until after changes have been made. The problem can also be partially rectified by replacing the Library.itl file with an earlier version (retrieved via the Time Machine backup), then remaking the desired changes to the iTunes library after postponing the backup procedure.