it's just my opinion, but not much. I've found that in those cases where I "know" there are upgraded rooms available but the front desk clerk says no availability, they are usually just doing what their manager has told them to do. You could try sending an email to email@example.com, laying out your case for why you believe there was availability, i.e., few cars in the parking lot, rooms for rent on the website, and so on, and who knows you might get some satisfaction. I did that once and was guaranteed an upgrade to a suite on my next visit to the Lake Mary Marriott. That was some time back, but I believe if we don't ask the question, we won't get an answer. Good luck.
I'm not sure about your experience, but for the most part I found that my "upgraded room" means that it's on a special floor, or is closer to the elevator. I have found that complimentary upgrades into suites require a great deal more effort on my part. (More frequent stays at the property and discussions with the staff). Also, I have found that the room upgrade benefit varies from property to property, and that could be part of your problem. As I said, I usually get a slightly larger room on a designated floor, but if were thinking that your newly acquired Platinum status gets you a much nicer room or suite, I can tell you that it doesn't. Hope that helps!