Hi,
Assuming that you are working with iMovie 10.1.4 on a Mac, I would try restarting your computer, and then launch iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. Sometimes that cures glitches, particularly after updating.
If that doesn't work, you can try creating a new project and then copy/pasting into it your media from the initial project to see if the exposure is correct.
Failing that, you can try opening iMovie in a new library, by closing iMovie and then re-launching it while holding down the Option key until a box appears that will give you options to select a library or create a new library for iMovie to open in. Then see if your clips import properly.
As a workaround, you can adjust the exposure with iMovie's color correction feature (the button that looks like a palate in the tool bar at the top of your screen).
Hope this helps.
-- Rich