"I checked in and was given my key packet for a room on the 6th floor. The front desk agent thanked me for being a platinum member and told me the lounge was on the 20th floor."

First problem is not being recognized as PP. Marriott makes the distinction and if the front desk does not or cannot tell the difference you have no way of knowing if they even know your status.

And PP (and I believe Plat) are 'entitled' to upgrades to the best room available, Marriott says so. We are not entitled to upgrades to suites.

Given that as WillTravel states they should say 'sorry Mr/Ms PP, there are no room upgrades available tonight. At least in this case they acknowledge your status and tell you they understand what you are entitled to and or expect.

Having had a few conversations with GMs over this type of issue I've found more times than not their staff are oblivious to PP vs. Plat. rarely have they said we can't do anything for you. In fact I was in CPH earlier this year and had this exact type of situation. ok room on a low floor, the front desk said that's all they had. The hotel was also sold out when I checked on line. After a quick conversation with the manager on duty, she offered me a choice of several rooms which included an executive room on the CL floor.

I think the most likely situation is that they filled up all the upgraded rooms with the Plats and even sweet talking golds who showed up and checked in before you.

The problem most hotels, IMO, is they don't know who their PP 'VIPs' are that are arriving each day. Seems that there are only a few at any property at any time so it would be reasonable for them to know who their 'best' customers are that are checking in each day.

Instead I think the hotel falls in love with the bus driver from the tour group that was booked into the hotel as a low room rate but with lots of rooms. IMO.