I have a nice picture that looks terrible after I scan it. There are a variety of possible answers. Assuming that your scanner is reasonable, the issues generally revolve around contrast and brightness. It has to do with something called the dynamic range of color and the way colors display on various devices and mediums. After many decades of hard work, modern film makers are able to produce a product of amazing resolution and dynamic range.
Color film can capture some of what our eyes can see.
Video monitors can only display a portion of what the film can capture. Color film prints can only show a small fraction of the information available on a video monitor. Color ink-jet printers are worst of all. Relatively speaking, they can't do much. Light passes through the ink, reflects off the paper and passes again through the ink on its way to our eyes.