In my experience, the frog actually has 3 targettable "object".
First, the Necklace which spawns jewel when hit.
Second, when it is looking left and right before opening its mouth and then launch an attack.
Third, When the mouth is open.
IF you threw and the 2nd target, the jewel will not hit the mouth.
I used to throw it at the second target and was baffled why it didn't hit the mouth. And I wonder why the second target even exist.
Just make sure to select the target when the mouth is open not before.