Out of balance batch - VAT not added to Trade Creditors balance

We are experiencing issues in Accounts Payable when we are posting voucher batches.  Sometimes we get the out of balance posting error because the VAT isn't added to the trade creditors amount in the journal. 

In the work centre the error message is the Intercompany out of balance message.

The tax rate/areas are all set up correctly and it's not the first time that a voucher has been posted against the suppliers which go into error so it's not missing AAI's. 

Does anyone know what could be causing this to happen?

Rebecca

Subject : Error: Intercompany Out of Balance

Cause . . . . . Your intercompany settlements are out of balance for