Sometimes when editing a shift you will see the following message on the right:
​

This means that hours have already been submitted for this shift in the time registration. When hours are submitted for a shift, no further adjustments can be made to the planning.

Do you want to adjust the submitted hours? You can do this in the time registration. Read more it here.

Note: The reason that you cannot adjust the shift anymore is because the schedule is an expectation and separate from the actual approved hours. This way you will be able to compare the expected hours on the planning with the actual hours in the time registration.