Q: Can not manually add album art

I've just got a new computer and installed the latest iTunes for Windows. I added my music library with no problems and was happy to see that album art was automatically updated however on the few remaining albums which have not had artwork added automatically I am unable to do so manually (right click album >get info >artwork). The art appears to have been added however when I hit 'ok' and return to iTunes there is no change, it still displays the grey double quaver.

I've done this many times before and have had no problems.

Thanks,

Alex