If your states are different sizes, you're only allowed to switch states if the new state doesn't cause the sprite to overlap solid ground. For example, if you duck to get into a tunnel and you're ducking through the tunnel, you won't be allowed to stand up. If you don't align the states correctly, you might be causing a larger state to overlap the ground you're standing on, which would prevent you from switching to that state. You have to select the right relative position parameter (maybe align the bottoms of the states when switching to make sure the bottom of the new state isn't below the bottom of the old state) to ensure the new state doesn't overlap solid areas of the map.
