While maintaining a wet edge in poster colour, a new stroke pushes the previous wet stroke. What is the likely cause?

Correct answer: Too much paint or water on the newly loaded brush

An overloaded brush can deposit too much paint or water onto the wet stroke, displacing its liquid and pigment and pushing the edge. Use a more controlled brush load. A flat surface, the same colour, or a broad brush does not cause this by itself if the brush load is controlled.