supplier bill attachments disappearing from sales register

sel_17

I attach my supplier bills to each transaction but when looking them up in Purchases register some (not all) attachments have disappeared!  So frustrating as we modify PDF's with important information prior to importing into the in tray.  Where are they going and how do I get them back?  

sel_17

I've done that but they have not reappeared. 

I don't know if this helps but the attachments disappear after "paying a bill", also the only ones disappearing are for foreign currency.