There may be a couple of reasons why you're having trouble with the edit tools. Pease review the below list to see if you're problem fits any of the below issues and follow the steps to successful editing.
1) I can't save my edits.
Be sure to click the blue checkmark that appears at the bottom right hand corner of your screen to save any edits you make. Tapping this before moving to another tool or image will ensure your edits are saved. Tapping this checkmark is essential in order to layer your edits and accumulate the improvements for your final scan. Otherwise, they will not be applied.
Conversely, tapping the grey X in the lower left hand corner of this toolbar will revert any changes you have made since your last save, allowing you to undo any unwanted edits.
2) The edit tools are greyed out and unelectable.
There are two modes with which to view your scans. You can toggle back and forth between these two modes at the top of your screen.
The left icon puts you in carousel mode for individual previews. The right icon switches you to thumbnail mode for selecting multiple scans at once.
Some edit tools cannot be used when you're in thumbnail mode, namely crop and skew. To use these tools, switch to the carousel mode by the icon on the left and the tools will become selectable.
In other cases, icons may be greyed out and unelectable because of the limitation of the function, such as trying to re-arrange scans if you only have one scan in thumbnail mode. The app disallows you from doing this because there's no need to have the re-arrange option with only one scan.