Fixing shape keys that don't work when using the slider?

Something has happened to a bunch of shape keys I’ve created for facial expressions. When I click on them in edit mode they appear to be in the model. But when I use the slider in object mode, the shape key doesn’t turn on. What could have possibly happened to break it? So frustrating…there’s been a few instances of doing this character work where I break stuff along the way and don’t find out until later.

Adding a new shape key works…but the old ones are not.