A missing stay request should help, but under Marriott this is a painful process. You need the reservation number and a copy (preferably electronic, scanned copy OK) of the final invoice. If you're missing any of the details, the Marriott rep will ask, via email, many repetitive questions until you get credited with the stay.

The main challenge with doing a missing stay request is that the number of requests is higher than anticipated so they'll get to you when they get to you. I did a missing stay request in June and it took until August to get it resolved. I did another one in September but I never heard back (other than the standard we're experiencing more requests than normal) despite multiple follow ups.