How to unlock “Selected lessons” marked as “locked” with lock icon?

When I am scheduling a resource, sometimes I encounter this problem:

Under “Use Lessons” I click “Select Lessons” and when I try to select “none” (because I only want to use a few chapters out of many listed) some remain selected and seem to have a lock icon on the right of the blue bar, beside the “change order” arrow connected with that chapter. I do not remember ever doing something that would lock them, and I cannot figure out how to unlock them so that I can then un-select them and have a fresh start.

I have had this issue both with resources I created myself, and ones others have created. Thank you for your help!