Each plate is really picky about the right item for it to go 'click'. What worked for me was a bucket of water, a book, the heavy barrel and a vase.
Yes, the plates only click at specific wights: 1, 2, 5 and 7.5 and the four plates are different sizes, the smallest needs 1 and the largest requires 7.5. There is a large barrel you'll find in the area, the only "Large Barrel" in the game and it conveniently weight 7.5....you can find it by holding down ALT or you can put 2.5 worth of stuff in any othe crates/vases/barrels around because they all weigh 5 when empty. For 1 and 2 I'm pretty sure you can find some junk items in your inventory to meet the criteria.
