How to: Make a slide follow a master again after changes have been made.
Solution:
Reapply the slide layout.
1) Move to the desired slide that is to follow the master again.
2) Select the 'Format' menu and select 'Slide Layout...'. (The Slide Layout dialog box appears with the current master layout highlighted.)
3) Click 'Reapply'. (The master layout is applied to the slide, overriding the changes that had made the slide different from the master.)