The exact same thing happened to me (random hotel in Palm Springs), the hotel can't help and it took me several chat reps until I found one that recognized what happened and manually applied the credit. Funnily enough, even after he manually applied it, my delta stays credit bar remained at zero, so it's quite possible I can double dip. Keep trying the amex reps, someone will be familiar with this as an issue and you might even get an unexpected bonus out of it.