This is the forum for helping you out if you're stuck in Grimrock 2 or if you want to discuss about the mysteries of the Isle of Nex. Warning: forum contains spoilers!
Even with your hints I don't get it. I've put items on every surface I can reach to no avail. As far as I can see, there are only two types of floor tiles: metal grates and white stone. No pressure plates. On which of them to I have to put an item?
Are you on the floor just after entering? Perhaps you are not at the puzzle we think you are at. IIRC at the floor there's a place where you can drop an item to a "round" plate below that may be difficult to make out as a pressure plate.