MORE tag not working. What can I try?

I am using the advanced wysiwyg editor in my installation (with Blix theme) but the MORE tag doesn’t seem to work.

It appears in the editor itself (as a line with more underneath it) but on viewing the front end, it doesn’t split the post. I have the full articles on the “index” page for that category

Looking for places to troubleshoot / things to try

many thanks