It seems random, but I am noticing tonight that there are some episodes of some shows won't let me re-edit commercials. It will open the window when I select re-edit, but when I've made my changes the button to save it won't work like it's greyed out as an option.
If I close the window and open it again, the edits I made haven't changed.
One specific show I am trying to do this on is "Building Off the Grid" from a TVE channel. Some of the episodes let me re-edit just fine and some don't. It seems random.