For the last few days, the normally excellently reliable SpamSieve seems to have gotten ill!! A virus perhaps? 
Here is the problem. OS now is Mavericks. SpamSieve version 2.9.11
Many “good” messages are ending up in the Spamfilter.
Log…checked… I see the message marked as “good”
Re installed the plug-in.
Any help appreciated.
This is probably because the plug-in wasn’t installed. Otherwise, you can check the log; the important part is not whether the messages were trained as good, but whether (or why) they were predicted as spam.
OK Thanks. It seems to be working now. Will keep an eye on it and report.