How is the weather at night? Sometimes cooler or damp weather conditions can trigger a minor drip or bubbling in the eyes. I usually see symptoms appear and disappear with a few of my chickens in the colder weather.
Also make sure you keep them up to date with worm treatments. A worm or...
It could be from particulates in the air, like dust or inhalation. Where are they sleeping or spending their time?
It could also be from an infection in their sinus region. The sinus is inside the head behind the roof of their mouth, and when it gets an infection, can cause eye discharge...