Hi Laynie. It could be a variety of things. Depending on the amount of tint in the colour it could be separating some and not staying mixed. The paint could be drying to fast if painted in the sun or in hot temperatures. It could be lap marks that you are seeing that are going into the paint that is drying. Pictures may possibly help. But ultimately what needs to be known to answer the question with a better degree of certainty is: Line of paint being used, application method, outside temperature/humidity/wind/sun/clouds, what time it is being applied, how many painters applying the coating to that particular area in what fashion.

But in my opinion for now would be that the paint is drying before they have a chance to paint out one board meaning some areas are dry or drying when wet paint is being applied over the drying when back-brushing it out.