I have finally discovered that my RSS reader (Feedreader) is reporting new forum threads but not existing ones that are updated with new posts after I have read them. 

So the sequence for any one thread is:

a) new thread created.
b) thread appears in Feedreader.
c) I link through and read the thread as it stands at that instant.

The thread will never appear again in Feedreader.  If I manually read the thread I may find new posts (that I have never seen before) and if I go back to Feedreader there is nothing despite the last update being after the date posted of new posts.