I would really appreciate some help.

In 2016 I bought a Dyson Vacuum with a Citi Hilton card. It came with a two year warranty. The manufacturer warranty ended a few months ago and the Dyson quit working about 3 weeks ago. In January of this year Amex bought the Hilton card and converted it to a Amex Hilton Ascend card. About two months ago I upgraded the card to the Aspire. I contacted Citi about the warranty and they say it's an Amex card now, file through them. I did and it was denied since it was bought with a Citi card. Wouldn't you think when Amex bought the card they took over responsibility for warranties?

Any idea on how I should proceed?