raid, at what resolution are you scanning? And what bit depth?
It's usually best to scan at higher resolution than your final size would be. Like, these images look like 900x600 pixels... so e.g. you could scan at a resolution giving you 1800x1200 pixels and downsize to 50% in the software you use for postprocessing.
On the bit depth: If possible scan in 16-bit grayscale (or 48-bit colour if it is colour) and do the postprocessing in 16-bit grayscale, NOT 8-bit. Gives smoother transitions between the graytones. I get the idea that you scan/process in 8-bit from the higher contrast images you show.