Master pages suddenly not displaying all items

[AAAARRRRRGGGHHH]

Hi there,

I have an Indesign CS6 116 page long catalogue. I have around 20 different Master page (spreads) all based on "Master A". The document has been working fine. Suddenly today when I try to place a master into a blank page it does not copy across all of the master items. The background image is missing. It shows on the master preview (edit) pages. But does not show up when I place them into the document. I have made no changes to file structure or directory listing.

I have overidden the master items to see if it will make a difference but it doesn't. This happens across all of the differing "Masters".

Can anyone suggest what might be going on here? Or any way of getting around it? It has happened at very inconvenient stage as I am almost ready to publish!!

[AAAARRRRRGGGHHH]

This is an update. It seems that if I place a master page into a spread that already contains the same master then it does copy across all items of the master that I drag in. But throughout the document I have mixed masters on spreads, why is this happening now?