Oops, sorry - the board has stopped going to each thread's first unread post.....now read that.
Yes I am finding that too. The "Show unread posts since last visit." link seems to have started missing out some threads. I notice when maybe a day or so later someone replies and I realise I hadn't read the earlier posts.
Just to confirm I have noted this report. Aware of occasional issues in this area in the past ... not new, but not consistent (or, rather, no consistent pattern I know of) so significant analysis required, out of proportion (at the moment) to other tasks - sorry.