Comments highlight remaining

A client is making changes to a report where three authors have added their opinions on making changes - these appear ok in the comments pane on the right.

In some cases, my client has read threough and decided no chnage is required. However, when she deletes the comment thread for it, it disappears in the pane but the yellow highlight remains on the page itself. She has tried deleting on the page but it won't budge. So it's making it a little messy to know what is needed when highlights appear but no instructions! She has resorted to adding 'No action' on each but it would be useful to know how to clear the comment highlight.

I use Acrobat Pro my end but the same issue is there - I can't take off the highlight either.

Any help much appreciated.