movie and tv show artwork is missing and not showing up in my library or when I stream to Apple TV

HI some of my TV show and movie artwork has gone missing in my library and when I home share stream to Apple TV.

My library is on an external drive and has always woked well until recently.

I can get the shows to play but the art doesn't show up on about 25% of my content.

I tried using 'Get Info ' And it shows the artwork in the dialogue box but not in iTunes or Apple TV.

Also some of the seasons have the wrong season's arwork showing.

Is there a way to reset the files to the original ?

I tied to download artwork but nothing comes in.

A few of my purchases are no longer being offered on the iTunes store.

I reset the cache a few time and relinkd the iTunes media folder to the correct location.

I have also restarted everyting a few times.

thanks in advance,

christos