Chickens missing feathers dont know what to do

[LadyChick828]

I am a High School student and have raised chickens for a few years with no real problems. Lately, several of our chickens have patches of missing feathers on their bottoms, stomachs or both. We feed organic feed and meal worms. There's no sign of bugs or anything. We have a large coop and very large run.
What could be the reason for this and what do I do?

 

[CluckerFamily]

Could they be rubbing their stomachs against their roost bars at night?