Looked at your file. There are a few things you need to know. Displacement maps are data maps so the color space should be set to Non-color Data. Since they are data maps, you should avoid using jpegs since jpegs are 8bit. For best results, always use 32bit images for displacements. Mid-level for all quixel displacement maps should be set to 0.5.
For some reason, using attributes for mixing doesn't work. Maybe it's a bug or some limitation. Maybe Juanjo could check that out.
Using procedurals to mix works fine.