How to check if my antispam filter is learning?

I'm using ZCS 5.0.12. All my users receives mails in russian. We always send spam to the Junk and false positive we mark as not spam. But I guess the filter is not learning. Every day i receive the same spam mails and mark them as Junk, but the next day I found new one in my Inbox (with the same message body).

Spamassasin is turned on, there are a lot of mails marked as spam every day. The ratio of true positive (spam marked as spam) to true negative (spam not marked as spam) is about 6:1 which is not good IHMO.

How do I improve the filter? We have spam and ham accounts, so it should be learning. May this be because of russian text in messages? May be SA can not analyse russian messages, just their headers?