[Koha] Problems with the renewals

[vanda koha]

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