I just answered a similiar question on the same problems I have been having:- 

"I have had several issues with Buyers telling me that they cannot pay. 

I have resent invoices and still nothing for them to pay.

After lots of investigation I have realised that after changing some of my postage policies, much of the information has "dropped" off.

My issue seems to have been the "postage option" 

I changed the invoice to "standard delivery" and all went through fine on the last problem item"