Resolution
According to the Spamassassin documentation the Bayes system is not activated until a certain number of ham (non-spam) and spam has been learned. The default is 200 of each ham and spam messages. It means that Spamassassin begins to filter mails (according to your trainings) after 200 spam mails and 200 none spam mails were learned. This statistics is needed to distinguish spam mails.
This default value could be changed in /etc/mail/spamassassin/local.cf by using the following spamassassin options:
bayes_min_ham_num 100
bayes_min_spam_num 100Spamassassin can be trained per mailbox basis on Mailname->Spam Filter->Training page in Plesk CP.