Photoshop :: Image Consistency For POD

It's the perennial problem with print-on-demand – inconsistent quality, particularly in the images. In the latest proof received, there are 24 B&W images, all quite similar in values, all 7x7cm. Despite all images seeming to have a very similar range of values, the proof came back with 4 pics looking absolutely brilliant (when one has an idea of what to expect from POD process) and the rest, not mediocre, but dramatically inferior. Blacks and crispness of the the four good images are excellent, while the rest are washed out with poor contrast.
 
When viewing the images on screen or printed out, the inconsistency in values isn't apparent (not that this is a method to judge what to expect from POD); the results are at least stable. I might hope for some of that consistency in the printed book, revealing poor or better quality across the board, when similar images are present. Perhaps the printer makes selective adjustments to some – which is to be checked, but until then, what are the fundamentals of optimizing images for POD?