Its not to do with the colour of the paint if its breakbeamed.
There are several factors that can make a difference however.
On an angel(and many other guns) the ball can be allowed to roll backwards in the breach.
Small paint or a bad ram position can aggrevate this effect to the point where the ball has rolled too far back to be picked up by the eyes.
Similarly bad ball detents can allow the ball to roll forward, causing the same problem.
To prevent this problem from happening on spitfire guns we put alot of effort into finding an eye position that works with even the smallest paint with a badly set ram. So our customers wont get that problem.
If you dont already have a softnose bolt then get one. Otherwise just use large paint.
If it was summer I would also take a guess that light may be shining into the gun somehow and messing with the eye signals. But I doubt sunlight is your problem.