6 acorns in a row- optional items in investigation 3

I feel like a bumbling idiot. I see all 6 acorns. 4 are already on the bench. The object is to place all 6 in a rown in order on the bench. 4 out of 6 isn't bad??? I found the other 2, but cannot place them on the bench. What am I missing?

I don't understand why these were called optional- were we able to skip them? I did not notice.

You need to watch the position of the cursor vs the position of the item. If you put the item in the spot and then click, the point of the cursor is not in the correct position...so, it's viewed as an incorrect click. Make sure when you align the item, the point of the cursor is in the "hot spot" area.
...this happens throughout this screen..so if you're sure you have the right item but it's not being accepted, double check to be sure your cursor pointer is in the hot spot.