Never had problem although sometimes it takes a few days to see them in your account. You can always go to website and post "Missing Stay"report. You might keep your Marriott bill/reservation details just in case they might ask you to fax or email your bill to office. Or, perhaps you could check your bill to make sure your points are posted,and/or call the hotel and ask if they have posted them. You can also sign up for email account of each stay on Marriott website.
I haven't done a lot of these promotions yet, but I was at the Crystal City Marriott and the front desk said they were going to add bonus points to my account for my stay...I am not sure why, but it might have been because there were not any great rooms available. It never happened though. I put a note about it on Trip Advisor and the hotel manager left a very nice note to contact him to get the bonus points...but there did not seem to be a way to send a note to him on Trip Advisor so I just let it go.
I did stay at the JW Marriott Denver Cherry Creek recently and they forgot to add 500 bonus points to my account as my Platinum arrival gift so I had to call Marriott Rewards Customer Service to get the points.