To anyone else who may be interested,
I finally realized where my problem lied:
I tend to consult the forum without bothering to log on.
Whenever I want to post something, then I log on.
When one is logged off, the "Subscribe to this topic" item is
not displayed.
When one is logged on, this and other options are displayed.
Answering is not the triggering event but logging on.
Very obvious, but had not thought of it.
Dan