This is due to the VAT registration code not being entered into each address associated with the customer in Dimensions. For instance, if there are three multiple addresses for one customer, each address needs the VAT reg number entering via the following steps:-
In Dimensions, on the Customer tab, enter the relevant customer code
Click Addresses
Select one of the addresses
Click Edit
Click VAT
Enter the VAT reg number in the VAT Registration field
Click Save
repeat steps 3 to 7 for the next address
In FactoryMaster, select Sales
Click Sales invoice menu
Select Export sales invoices to dimensions
select Export History
Select the relevant batch number and click Export.