Plasticizers are chemical compounds (typically pthalates or adipates) that are used in the manufacturing process of materials containing PVC to soften the PVC. When materials containing these chemical compounds are of poor quality or are old, or have been stored under the wrong conditions, the plasticizers leach to the surface of the material. Given that plasticizers are oily (hydrophobic) compounds and HP Latex ink is water-based, the presence of the plasticizer degrades the adhesion of the ink to the substrate. Plasticizer shows up as randomly non-uniform areas on the substrates with “coalescence like” graininess and increases the visibility of pinchwheel marks.
To get rid of this you need to “wash it out” with isopropyl alcohol. This is not practical for printing but can be used as a diagnostic tool: by washing only part of the substrate the defects can be made appear (not washed areas) and disappear (washed areas).
Otherwise, otherwise you need to change printing material or, at least, the production batch of the material used (preferred the ones that have not been stored for a long time).
In other to give a solution to this problem HP Latex 560 and HP Latex 570 printers have released a "wiper roller" that pretty-much eliminates this issue, allowing materials with plasticizer migration to be printed without a problem.