Found the reason for this.
If you are in the case where you have a sound file playing against a still or video image and you are placing both elements in seperately you may find that when dragging in the still image you get a dialouge box asking for the duration of the image…
I’ve just discovered that this acts a bit too strongly and that duration set will remain regardless if you change the elements in that timeline.
The fix that I know of is to scrap the offending timelines and re-do them again.