Data scrubbing ... ah, the joy.
I would suggest that the failure to match the fields likely comes from some errant control characters that were present in the source data (either on the parent or child side) in the original Excel spreadsheets. You might try creating a new, parallel set of fields (just temporarily, so you don't mess anything up) and use the Filter function to strip out anything that doesn't conform to strict alphanumeric characters. Something like this:
Filter ( __fkMemberCity ; "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789" )
Just use Replace Field Contents to populate the new fields, and see if the join will work then.
Can you upload a copy of the database? In order to attach a Zip file, you have to click "Use advanced editor" in the upper right of a reply window. If it's not related to a need to scrub the data, someone will be able to see what's causing the problem.