A lookup copies the data in table A::field A into table B::field B when there's a match in the key field between the two records. But it doesn't relookup a key field because there's no valid match between the records.
If you want it to do that, you need to establish another relationship based on a different key field and use that relationship for the lookup.
Did I understand your problem correctly?
I have a relationhsip ::Name to ::Name with a lookup between them; and I can't do a relookup on them to fill in the fields of the 2nd (dog) table.
Don't copy data unless you need to (e.g. prices for invoices).
If you have a table Owners, and a table Dogs, and they are related by a key (which should be an auto-generated key of some kind), then on a layout you could display the dog's "full name" using a merge field like <<dogName Owners::nameLast>>, or just put the related name field itself on the layout, or refer to the owners last name in a calculation via that relationship.
As Mike said: if you use the name itself to establish the relationship, and change that name, you invalidate the relationship.
This is one of the reasons why you do NOT want to use names or other real-world attributes as match fields; use a serial key you define once and that has no inherent meaning except identifying a record uniquely. Then you can change any of the parent record's attributes without breaking any links to existing child records.
Thanks. I think I have it now. I put it as a calulated value of the client last name.... and some jerk (*blush*) put the key field as _clientIDfk but the data for the clients was in the _classIDfk.... (oops)