I have found a couple of reasons. If you book in home resort they remain locked so it can’t be modified with non home resorts points.
If you modify a reservation made during home resort priority to something else, the contracts will remain locked most of the time.
The newest one I discovered yesterday is when you have restricted points as part of your membership. I booked RIV for January using my SSR points as I wait for my new resale RIV to show up to swap the points.
But my BLT restricted point contract I sold is still there. The SSR is locked but the message just says those points have to be used, so it’s related to it being RIV
In cases where I wanted to swap points around for a trip made 7 months or less, I have called or emailed in order to get them to do it for me if I couldn’t get the rooms making a new trip myself and then canceling.