This level is a long one, clocking in at around twenty minutes. I had to do three tries at it before I was able to complete it.
There are around six places where you have collect orbs. This requires you to fly into orbs that then each fly to a central part of the section you are working on. When you collect all the orbs (anywhere from five to ten for each of the six sections), one feather for that area becomes available.
In my two unsuccessful runs at World II, there were orbs that I flew through, but they did not fly to the collection point. When the orb did not make it to the collection point, it did not respawn, causing me to have to restart this level over. It's frustrating when you are one feather shy of finishing the level and the orb glitches and you can't complete the level.
My suggestions for getting through this level:
After you fly through an orb, watch to make sure it makes its way to the destination point. When each orb is successfully collected, there is a bright light at the collection spot. Some areas are large and the orbs take a while to get to where it needs to go.
If you are flying through orbs quickly and there are two or three flying to the collection point at the same time and you happen to smash into something, orbs may glitch and not get to the destination during your respawn.