My elderly parents flew STR-JAC and return. Booked through KLM, first two legs on KLM, last leg on Delta. The KLM flight to SLC was delayed (no weather or ATC at the time, just left the gate late), and they missed their connection to JAC. It was the last flight of the day, so they were put on the next day's flight and given a hotel room in SLC. They booked Business, but flew in regular economy on the last leg. KLM (who caused the misconnect) forwarded the claim to Delta, who gave the following response: "...Regarding your seat downgrade, I have forwarded this to our Refunds Department for proper handling. You will receive a separate email from them once reviewed and validated. Additionally, I am issuing a Electronic Credit Voucher for you and your companion in the amount of $xxx each."
They received the E credit voucher, but won't be able to use it (too many misconnects on this trip stressed them out too much, and they won't fly TATL ever again). They never received the downgrade compensation, and aren't sure where to claim that, or who to contact.
Neither Delta nor KLM acknowledged the delay (~18 hours), or paid the compensation due. How can they escalate this?