It's hard to tell for sure from the scanned images, but to me it looks like you are slightly underexposing. You don't seem to have a lot of detail in the shadows. I think this prevents you from printing any areas with a true black, and will keep your contrast looking low/muddy.

As far as filters go, others have given you good advice. I don't see that they would make much difference in the three images you presented. But for images where a contrast filter will give you a strong effect it's very hard to reproduce the effect in post (either scanning or printing). So you should certainly learn to use filters and use them where appropriate.