Sounds like the nut behind the tripod needs adjustment. Everyone does it, close down instead of opening up, vice versa, etc. Basically, if the photons from the dark areas of the scene didn't hit the film enough, no development increase will help - you can't develop silver halides that weren't exposed. If they did receive some exposure and the scene wasn't too contrasty, you'll improve things by increeasing development. If the scene was contrasty, you'll still build up more density in the lower areas but the higher values will increase proportionally more and you'll end up with a negative with a lot of hot spots. A proportional reducer would help here. So would masking. If these are really important negatives, the best thing would be to test this. Setup a scene at home with the same contrast range, underexpose by one stop and expose several negatives this way. Then, develop each for increasingly longer times, like +20%, +40%, +60% and see how the densities build up and make sure the total DR is within the capabilities of your paper. I've done this a few times and it really saved my bacon by taking the guesswork away from determining a development time.

-Mike