negatives dull?

Yes not enought light could well be the case or you need to change your default settings for scanning to make them lighter. If this isn't possible that just lighten the image using photo editing software.
Unfortunately the "photo" flatbed scanners are a big compromise for film negatives and slides and don't come up to the higher standards of a dedicated film scanner. If you've a lot to do it may well be worthwhile investing in one of these and Plustek make some good quality ones or for a greater cost Nikon do as well. Don't go for anything cheaper as the result will only be as good as you're getting now.

Stephen
July 2009