The system seems seems to have a problem remembering that I have clicked Subscribe under the options for posts of interest to me, I do get a confirming message of the action but in most cases the email advising of responses does not come.
I do get some e-mails though.
If you make a post, then you are automatically subscribed to that thread.
As to any others, I will mention it to a moderator.
As well as subscribing to Topics you've posted in, if you are interested in a particular Board you may Subscribe at Board level to it.
Failure of email notifications has been an on-off problem on the forum for a long time. It's been mitigated somewhat since the introduction of live notification feeds.
The Notification Feed is to be found at https://community.bt.com/t5/notificationfeed/page as well as being linked from all forum pages & the home page.