[Koha] Can't tick "item due" box in Patron messaging preferences

[Sonia P.]

Reading the wiki and plenty of old enquiries from other people, but I still can't find what's wrong with my settings...
Tha's what my "item due" line looks like (not the other lines!  I just found that screenshot on the internet, it's not from me):
http://content.screencast.com/users/oleonard/folders/Koha%20Screenshots/media/12d9e155-8156-4937-b722-ad616269004d/patron-messaging-preferences-column2.png
I have just reall all the system preferences and all the administration block and about everything ever written on koha notifications...

If you have any idea, I wouldn't mind trying about anything.

Cheers,

Sonia.