As each successive clump modifier has to have a higher density that the previous (clump points) by the time you get to level 4, the texel resolution of the map could limit how small the clump cells can be. It may be that the clump cells are smaller than the pixels in the map. TPU is defined as Texels per Unit area (cm) so you may need to increase the resolution of the map. If you have a region map, you'll need to increase the TPU on that as well, as the region map resolution determines the clump map resolution, so that region boundaries and clump cells pixels will match. If you don't have a region map, you'll need to increase the resolution of the first Clumps maps for the same reason, as subsequent clump map resolution has to match that of the first. You'll need to regenerate maps for all the clump modifiers, though you don't need to regenerate points as they'll already be saved.
If you're just using the default resolution of 5, you should try 50 or maybe higher till you get enough resolution in the maps to define all levels of clump cells.
Cheers
Michael Todd
XGen Product Owner and Designer