There will be a discrepancy between what is on a computer screen versus what is on the printed paper. CMYK printing will always come out darker in the shadows than what you see on screen. RGB screens are always going to be brighter than a printed piece.
Computer monitors emit color as RGB (red, green, blue) light. Although all colors of the visible spectrum can be produced by merging red, green, and blue light, monitors are capable of displaying only a limited gamut (i.e., range) of the visible spectrum.
Whereas monitors emit light, inked paper absorbs or reflects specific wavelengths. Cyan, magenta, and yellow pigments serve as filters, subtracting varying degrees of red, green, and blue from white light to produce a selective gamut of spectral colors. Like monitors, printing inks also produce a color gamut that is only a subset of the visible spectrum, although the range is not the same for both. Consequently, the same art displayed on a computer monitor may not match that printed in a publication.
Monitor calibration also plays a factor. A screen may be overly bright, thus making the prints appear overly dark in the printed form. The printers we use are calibrated daily to a set point that produces great results across the many orders we receive, and we cannot adjust lightness on any particular order as we are running thousands of sheets through. The only way to adjust the lightness of the output is for the customer to adjust the original images in Photoshop or some other photo editor by bringing up the brightness or black point by about 10%.