Originally Posted by Dave Noble

When you book an AA codeshare, you are not booking with JL and your arrangement between yourself and AA is nothing to do with JL

If JL has agreement for seat selection on an AA codeshare, then any web related issue is AA's.

In many cases with codeshares, there can be a booking reference for the operating carrier and may be used for seat selection, but this isnt always the case

If you booked a JL flight, then you would not had any issue with seat selection - avoiding codeshares will avoid problems of codeshares - hopefully there was a good reason not to book the JL flight