You have edited a master (e.g. changed color) and clicked ”Update Instances”, but the existent shapes on the diagram are not updated.
Make sure that the function ”Match master by name on drop” is not activated (right-click on the master, Edit Master/Master Properties).
If this function is activated, it can lead to the problem that shapes cannot be updated anymore. Please deactivate it for this process.
Then open in Visio menu Developer / Drawing Explorer Window.
and delete the old master, that has caused the problems.
Then click ”Update Instances”. The shape will be updated.