Anyone had problems with United Mileageplus flight credit?

I flew LHR BKK on Dec21. When I booked the flight it wouldn't accept my Mileageplus number, so I had the agent enter it at checkin.

My boarding pass has the United member number on it.

When I looked at my United account there was no credit. When I submitted a claim online the response was:
"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."

Any ideas?