Tomsmom, welcome to the Fish Pond. This game is quite cursor sensitive. I found it's the point of the cursor, not the item you are trying to be place, that is the key.
...Click to pick up an acorn ... remember where you got it. Move the cursor over the bench. You'll see shadows appear which will be in the shape of the acorn. Look for a shadow that approximates the size of the acorn. Put the tip of the cursor on the shadow and click. If it's the right location, the acorn will stick in place. If not, the acorn goes back to it's original position. If it didn't stick, pick up the same acorn and try in other locations. I worked one acorn at a time.
...Good Luck