Hi
You must unlink them first.
e.g. Drag and drop (or add in the normal way) your clip to the timeline.
Right click either the Audio track or the Video track and select to "Unlink from Video" or "Unlink from Audio" depending which track you have chosen.
This will separate the two tracks. Both can now be moved about independantly.
(Note that if you choose to split either of them and they overlap then both will split at the cursor point.
If you want to split them indpendantly use the options provided from the down arrow to the right of the split button.)
Clicking the Lock button will only lock the track selected as shown by the cross hatching. If the video and audio have not be unlinked both tracks will show cross hatching and both will remain together.
If you have unlinked them, items on the selected track will remain fixed in place.
e.g.
If the audio is locked the unlinked video can be moved about.
If you lock the video track, the unlinked audio can be moved about.
Nat