[Koha] Problems with the renewals

Hi everybody, I'm having some problems with the renewals. When I make a loan the date is automatically calculate by the system (30 days for books, for instance), but when I make a renewal I have to put the new date due myself, because the system does not do it automatically: it actually assumes the date of the day the renweal is made and not the new date due (that is 30 more days) We are using 16.11.04 Koha version. What did I do wrong? Many thanks in advance Vanda