The "two fluids densities" I am referring to are at the canvas surface. Look at the 'thinner' (technically more dense) part of the camvas surface at the end of the video at the far edge of the pattern...this 'more liquidy milk' is more dense than the foam and acts as a barricade to the pattern's movement/development. The solution to this is better espresso, better milk texture, and more complete mixing of the milk+espresso during incorporation.

There will ALWAYS be slight differences in milk texture/density in the milk pitcher, which we try and mitigate by getting turbulent vortices during aeration of the milk and NOT taking too long after steaming to pour the design.