Make sure to enable camwera settings on your imported files. Some cameras deliberately underexpose images to protect highlights and then correct for that in the jpeg preview but not in the raw data. There a correction is written into the settings that Lightroom doesn't read by default. Using camera settings for the raw file allows that to be read for some cameras (but not all). You enable this in Lightroom Preferences -> Presets (Classic) or Preferences->Import (Lightroom Cloudy). Also make sure to calibrate your display. Many problems with too dark previews in Lightroom are actually caused by bad display calibration.