I am assuming that the relationships shown in what must be an early screen shot or an edited image are still those in use for the table view screen shot. (I should see other data fields like "FullName" and "comments" in the relationships image.)
I see leading zeroes in your customer ID--not how I'd set up a primary key, perhaps one customer ID field is of type number and the other of type text. With leading zeroes, I don't think the records will match. The relationships shown are correct, so if the data isn't showing and you can find a record in your contacts and/or notes tables with what appears to be a record of the same customer ID, then there is some nonobvious difference in the data in the match fields keeping them from matching. If they are both text fields, check for things like an added space.
PS. I'd make each customer ID field in the Customer table an auto-entered serial number in a field of type number. If I need to display this value with leading zeroes, I'd use a calculation field defined in the Customer table to format the data in that form.
If your customer ID values come from some other source outside of FileMaker, I'd make them a "customer number" in a text field defined in the customer table and use a separate auto-entered serial number as the primary key for the customer table.
I've now managed to fix this!!!!
It appears I had been given duff data. The CustomerID in the Contacts and DiaryNotes Tables had spaces at the end of them (ie '01234567 ') and the other tables didn't.
Once I'd sorted this out in the Excel file i'd imported, I readded the data and this is now working okay.