I have United Mileageplus as my primary mileage program.
I traveled SFO-DEL on Air India and the mileage was credited correctly. It was a return ticket.
On return, I changed the date and even the flight number (from AM to PM flight) and it seems Air India created a new ticket and did not carry forward my mileage plus number.

Now when I request miles from United, they say "Denied. Our records indicate that mileage for this flight has already been credited to another airline’s frequent flyer program. Therefore, this flight is ineligible for MileagePlus credit". I do not even have a mileage account with Air India to the best of my knowledge, but maybe I clicked something somewhere (pretty sure I did not). They do not have any option online to appeal this and get this sorted out. I have both the e-ticket and the boarding pass with me and will not dump them as they may need proof.

Appreciate any advice on how to sort this.