EFI ES-2000 Spectrophotometer

hi I'm using a EFI ES-2000 Spectrophotometer to create a calibration curve for ctp plates, the problem is that the unit is giving a significant lower solid density reading than is actually being printed, is there setting I'm using wrong or possibly something else? (I'm using a separate denso and pms book to achieve to k- 1.7,c-1.4, m-1.4, y-1.3 but the es-2000 says its k-1.3, c-1.2, m-1.2 y-1.01.

any clues?
cheers
frustrated printed
jet screamer