you have to spam left/right arrow key really really fast to begin the struggling during green bar. And because the movement of the bar has momentum now so at the beginning of your struggle the bar won't increase but merely slows down to a halt, only then it would begin to increase. This can give you the false impression that spamming keys does nothing as the bar doesn't increase immediately.
Also the jellyfish is invulnerable to melee attack. You have to trample on it like in SuperMario.