For some reason I cannot create a relation between two tables in my database by dragging the field name to the related table field name. Table are invoice and customer tables, with cust_num being a common field and the key field in customer table.

Any idea why this would all of a sudden be a problem (not just these 2, but won't work with any tables in any databases I have set up, even though it did before) and any work arounds or ideas for fixing? Many thanks in advance.