Forms :: Combo Box - Search Function Not Working Within Form

I recently created a database to be used to store incoming invoices. We have 4 suppliers which supply the same material and wanted each supplier to have its own database.

I began by making a database which included all the key tables, forms, queries and functions I would need to then save 4 copies, one for each supplier. This just meant I would not have to repeat the process for each individual supplier.

The problem is all my data is external and upon importing it into the database it has made the search function (combo box) not return results.

I believe this is because:
1) The form was created before the records existed
2) The records were imported from an external source and not created using the form.

However, the way I have designed the form is so I can see all relevant information linked with a specific invoice. This aids in gathering information quickly about an invoice but also so I can input credit note and invoice query information linked to a specific invoice on the same page.

Is there anyway to remedy this?

Or use a similar search function that returns records in my form "Main View"

The last thing I want to do is have to create the form again, although I don't think this will work either as reason 2) states.