This one puzzled me for a bit too, but there actually are enough stones. The trick is that the cannons that move will fire once from each position. For the upper cannon, that includes the position that it starts in even though there is no skeleton to hit from that spot. So move the upper cannon to the left position and fire it to get one more stone.
BTW, those are actually shells rather than pebbles though it is hard to see on a phone screen. I only point it out because it helps to know that when a game character gives you a hint about shells.
