some of them are slightly underexposed
So exposure you want me to keep my lense open a bit more?
Or up your ISO. Or enlarge your aperture. Any of those will add more light. Your call.
Shutterspeed, ISO and aperture are the holy trinity of exposure. Knowing which to adjust when, or rather, the impact on the image of adjusting each, is part of the art of photography.
Increasing ISO can add noise. Increasing aperture will reduce depth of field. Longer exposure time (shutterspeed) risks introducing motion blur.