There are different rules governing listing input and modification in Smart Matrix than were present in either of the old Matrix systems. Some fields accept fewer entries than before, some fields are now required that were optional when you entered the listing, etc.
When you go to make a change to a listing that was entered in either of the previous Matrix systems, Matrix will validate all fields (when you click Submit Listing) and ask you to fix any fields that are not in compliance with the new rules. This means that you may be trying to close out a listing, but you get an error that you have too many selections in the Nearby Amenities field; or you may go to change a price and Matrix complains that you need to enter something in the High School field.
You will have to correct any fields that Matrix is complaining about first before you will be able to make the change that you set out to do in the first place.