Spam filter not working

I’ve been wanting to report this failure for a long time, but in the end I’ve never been cheering until now.

moving undesired messages to the propaganda folder or marking them as such in my mail client (Kmail) does not seem to serve at all: emails from the senders marked as undesirable in the inbox are still entering.
I have tried to do it also from the web interface (moving to the junk folder so that the program learn, and mark as spam with the provided button) if it could be due to a fault in Kmai, but no, the result is the same.