Some paints are actually stronger than others. Sand can be strong and takes just a little to make it blend. Each paint is not the same. You can't blend from the lower paint, it has to be the top paint starting low in Opacity to blend over another.
If you try blending with a lower level paint it just makes it more blotchy (because it removes paint, it doesn't blend) but it gives it a great look for some areas.
One paints opacity isn't the same as another... To me that looks like the paint is removing layers to show. Blend with the top layer. :)
**It would be best if you expanded your paint layers for me so I could see it better. :)
**Right above the Brush area is a point where you can grab and lower to expand and show the paint layers. :)