More details are needed. Logs? Plan details?
It will only work from within the current plan period. If you click on a date after the original plan ended, you won't get an option to adjust.
I've just come to report the same thing. Surely this is a bug.
And 2 years later...it still doesn't work.
Go to the first day under your list of assigned readings, for example whatever "tomorrow" is (where there is no red exclamation point) and right click, and select "adjust plan from here"
And 4 years late...still doesn't work.
THIS WORKED. ITS NOT A BUG. We just don't know how to use it.