Newly Engraved Surface
This is a picture of a newly engraved anilox surface.  The cells have consistently thin walls and well-defined cell bottoms. 

Anilox Cell Plugging
When anilox cells become plugged, dried coating residue diminishes the capacity of the cells and causes volume loss.  In this case, you will notice that the thickness of the cell walls has not changed but the bottoms of the cells have far less definition.  An on-press cleaning with a chemical cleaner and roll cleaning brush or an off-press mechanical deep cleaning should remove the debris and restore cell volume.

Anilox Surface Wear
As the anilox surface wears down, cell capacity is diminished and cells cannot carry the proper volume of coating.  Under a roll scope, you can see that the cell walls are getting wider which indicates that the cells are becoming shallower.  In this case, re-engraving is necessary to regain cell volume.