1. The map was created anonymously without a password
Maps created without an account and a blank password are open to everyone. The issue could be that the map had no password to start with. It should be noted that if an anonymous map is saved by someone who is logged in, the first person to save it will essentially "claim" it. Maps created under an account without a password can only be saved by the original creator.
2. The Password was changed
If the map was created by someone, they can change or remove the password at any time and this would limit your ability to change it.
3. The Map ID Already exists
When manually setting a map ID, keep in mind that someone else could also be using it and if they set a password or it is associated with their account, you will not be able to save it.