spamassassin don't works some times ?

[Sylvain Blanc - CRI du Pays De Gex et du Bassin Bellegardien]

Hello,

I have a linux debian woody with
mailscanner 3.27 and spamassassin 2.20

generally it work fine and i have a response like that
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=-3.1, required 5,
IN_REP_TO, EXCUSE_16)

but some times i have the repsonse
X-MailScanner-SpamCheck: not spam, SpamAssassin ()

Which can be the cause ?
I have nothing in my log

Regards,
Sylvain B.