iTunes looses artwork from NAS stored media

iMac 3.2GHz INtel Core i5 1TB

OS X 10.9.5 3.2GHz

iTunes 12.1.0.5

EMC NAS Box/ 8TB

iTunes regularly looses my artwork for movies and albums. However if I right click and select "Get Info" it is there and re-appears. With thousands of albums it is very tedious to go througheach one, one by one and select "Get info" etc.

I have found a script online tha re-embeds the artwork however this is becoming a regular task as it still looses it.

There is no consistency as to whichalbums/moveis loose their artwork.

Everything is stored on a NAS box and I think this is part of the problem