Comment associated with a cell
 

I have a spreadsheet which contains monthly data with many cells annotated
by means of a comment.
The workbook gets copied to produce the next month with data being deleted
ready for new input. The comments continue to apply but do from time to time
require amendment.
Lately I have found that calling up 'edit comment' does not present the box
for editing - I have to 'show comment'
to be able to edit it then 'hide comment' to make it not visible. This
problem does not occur with all comments
in the sheet and I am at a loss to know what controls it.
Can anyone enlighten me ?