There are two possible reasons why Pitch Lock might not be functioning as you expect. First, it’s important to note that pitch locking is set for a given media source at the time the source is opened. (This is a technical limitation.) So if you change the setting for Pitch Lock in Preferences/Options, you will need to use the Reload Media menu item to re-open your media with the new Pitch Lock setting.

The other reason Pitch Lock might not be apparent is that adjusting the pitch in real-time is a fairly intensive process, and gets more difficult the farther away from 1x speed you go. If QuickTime gets to a point where it can’t adjust the pitch and maintain fluid playback, it will stop adjusting pitch.