What tables and relationships do you have?
You need a relationship that uses the "bill to" field to link your invoice record to a record in the customer info table.
What data is entered into the "bill to" field when you select a customer's name?
The templates are set up to display the customer's name, but enter their ID number. If you have a relationship that matches records by ID number (and you should), but your value list is entering the customer's name, then you are no longer matching your invoice record to a customer record by ID number and this will kep any customer information from appearing on your layout when you select a customer.
Thanks so much for responding. I think I've got it sorted. My ID number wasn't appearing correctly but now that I've deleted it and put it back in it's full steam ahead. Thanks for the help!