When you go into the Develop module with one of these underexposed photos, are the sliders in the right panels adjusted from their "zero" position? Can you press the "Reset" button and the image goes back to a more normal exposure?
If that is the case, then you have have inadvertently set Camera Raw defaults To something undesirable. Look under the Develop menu, and choose "Set Default Settings," and choose to restore Adobe Camera Raw defaults.
If that isn't the problem, or doesn't solve the problem, please come back with additional detail and screen shots.