Hi!
I'm not sure what "CustomerName" contains in the Order table. An ordinary relationship between Customer and Order, would be a numeric PK (Primary key) in the Customer table (Your CustomerID), and using the same field (with index type Yes - duplicates OK) as FK (Foreign Key)in the order table. The usage of "CustomerName" as the name of the FK, makes me believe this might be an issue here.
Only one other comment to the structure, the field Date in your order details, should be renamed, cause Date is a reserved word in Access (the Date function).
If what I've commented on above, is OK in your structure, then we'll have to move on to how you've buildt the forms (which fields are included in the forms recordsource. I think one thing to check, is which field do you store the selected customer in - is it the field "CustomerName" from the Order table or the field CustomerID from the Customer table) - but then, do elaborate a bit more on the form structure (main/sub, usage of combos...)
HTH Roy-Vidar