First, check to see if your board filter is on. If you have a board filter on, your filter is dimming or excluding your missing cards.
You can fix this by turning off the filter:
- Click the filter icon in the upper right of your board to open the filter panel.
- Then click Off.
Turning off the filter will make all cards on the board visible, and none faded.
You can read more about filtering cards on the board here.