In UserCP -> Edit options -> Default Thread Subscription Mode: (dropdown menu) will change your default behavior. You also have checkboxes for PM's email notifications in there too. If it is set to "Do not subscribe" or "no notification" then that is why.
I think it was on by default at one point and we turned it off for some reason...i forgot why now. But each user can customize it.
You can subscribe to forums, as well as threads too. So maybe you have admin and mod subscribed to, but not threads you posted to.
Otherwise it could be the email server. Which we are still using SMTP, and there is a limit per day for the free option. The larger the forum gets, the more chance at one point we are going to max that out each day. EDIT: now thinking about it, we might of turned it off for default to reduce email traffic, as we would cap that limit easy if everyone had it set to on.