Why isn't a join between two nicknames of the same data source being evaluated remotely?

You can check elements of the join operation, join predicates, and the number of rows in the result to evaluate why a join between two nicknames of the same data source is not evaluated remotely

Areas to examine include:

Join operations. Can the data source support a join?
Join predicates. Can the join predicate be evaluated at the remote data source?
Number of rows in the join result. You can determine the number of rows with Visual Explain. Does the join produce a much larger set of rows than the two nicknames combined? Do the numbers match reality? If the answer is no, consider updating the nickname statistics with the SYSPROC.NNSTAT() stored procedure.