Songs and books purchased downloaded and used appear grey in listings on touch and are not available for downloading on iPod touch. No cloud symbol. Not able to reload from iTunes. Just in lists in grey. Space on device ok. Help!! Please.

My mother really loves her iPod touch, but it has developed an issue I cannot get to the bottom of.

She has purchased music videos and books from iTunes and downloaded them. She has played and use them.

A small selection of these files now appear greyed out on her iPod touch. There does not seem to be any consistency with names as to a reason for corruption. They do not appear to be downloadable again. There is no cloud logo. There is no downloads in process according to her copy on iTunes or her iPod touch. There is sufficient space on her device for these downloads.

I am not sure where to go next with this… Any sensible suggestions?