Value for the flexfield segment Supplier does not exists in the value set

I created a DFF in the Item master screen to select suppliers and linked it to a value set based on a table called "PO_Purchasing" to pull the vendor_Name.

I am good with some vendors when I try to link them to an item, however, for some vendors I am getting this error:

"Value  <vendor name> for the flexfield segment  Supplier does not exists in the value set <value set name>"

Any clues??

Thanks