United Platinum member cannot obtain double status

Hi, I am a United MileagePlus Platinum member. I tried to link my United and Marriott Bonvoy accounts at

https://rewardsplus.unitedmileageplus.com several times for the past couple of months. Each time I get a message on the screen saying it would take about a week, but nothing else happens after that and I don't get a confirmation email either. I called United a couple of times, each time they say the problem can be fixed by Marriott. And each time I call Marriott, I get a response saying the problem can be fixed by United. I am about to give up altogether but before I do, I wanted to give it one final shot by posting here and hoping that someone can help. Thanks.