Two thoughts here.

First, given you have one lab that works and one doesn't, talk to your labs, it may simply be that one cares and one doesn't. They should also be able to tell you if your negatives are over exposed or under exposed.

Second, with regard to scanning questions and all that digital color balancing jazz, APUG isn't really the best source, in fact it's off topic. may be a better place to ask. has tutorials on scanning too.