Originally Posted by Bowgie

Here is a suggestion, which isn't specific to the OP's I vs J problem or MU. The trick to avoiding pain and frustration on partner flights is to get the ff number correctly assigned by the partner airline so that Delta processes it without human intervention. Having the Skymiles ff number correctly printed on a partner boarding pass is no guarantee success. But keep the bp to claim miles in any case.

Get the reservation number. Get the partner ticket number. Go to Garuda Airlines website and find the "Check your flight" screen. Enter name and reservation number. That search will fail. When the screen refreshes, it will have a box for name and now ticket number. Enter the ticket number. It will pull up your partner flights, even if not on Garuda. When I tried this for a recent China Southern (CZ) ticket, it had my correct Skymiles number, but CZ's ff program. This is despite having CZ add it to my reservation as Skymile (not their miles).

The Garuda website screen has pick box around the ff program name. Change it to Delta Skymiles. Save. You get a confirmation e-mail from Garuda. I think the overwhelming problem is that our ff numbers are getting assigned to the partner's ff program, which is why Delta says "ineligible for credit" initially. At least until they can verify no partner miles were earned.