A couple of questions. Were these images made straight from the camera card or had they gone through some imaging software. If it's the former then it could be that they were underexposed, hence they look dark when printed. Many monitors nowadays are very bright, in most cases too bright, and this can give the impression that under exposed images are OK. Also if you used some software to make adjustments, then again an over bright monitor can give you the impression that the images are OK when in fact they are too dark for printing.
Have a look at the histogram if you can. If the curve is towards the left of the graph, then I would suspect it's exposure. ( This can depend on the subject though). Otherwise my suspicion is with the monitor brightness.
