I have been a member of Loyalty Marriott Rewards for many years accumulating over one million points, 41 nights in the last year. Unable to secure one room for two nights in Amsterdam in any of the three properties in September? Don't believe I will be a loyal participant much longer!
I fear that you are experiencing a problem that occurs not infrequently in Amsterdam - and most particularly in the first weeks of September. The general problem is a shortage of mainstream hotels, and hotel rooms, especially in the centre; the causes lie deep enmeshed in a confict between increasing demand on the one hand, and on the other, a wish to protect the most traditional parts of the city from what are seen as the depredations of tourism. The specific problem is a major conference in September which effectively monopolises the supply of rooms both in the city itself and its immediate environs.
I'm sure you won't feel any better for reading this, but at least it might help to explain the situation a little. I too have been caught in this situation - and ended up staying in Brussels!