Hi,
I still have this problem for a long time now. Example here: https://forums.chaosgroup.com/forum/…491-cactus-rnd
jonnybefree replied, but I didnt receive any notification about his message. so I wonder how many messages I overlooked because of this.
Likely, many.
The platform is a bit broken, and i believe some bits are beyond us fixing them (and not for lack of trying either.).
Notifications are one of those parts. 
I think this is a big problem. Forum is mainly for asking and problem solving. I wrote a lot of problems or ideas, but mainly something just doesnt work properly and never get an answer. And now I never will because I never find those threads after so long time.
Mail notifications do not work (sometimes promptly, sometimes at all), while in-forum ones do.
My own solution is to keep the forum page pinned, and check back as often as i feel like. 
Ditto. I just go to the top corner of the forum and click my icon and go to my recent posts, all of my threads are there, the ones in bold have new messages. I’ve always used it like that.
I mean web notifications, not mail one. Someone replied to my post and notification should appear, but it doesnt.
Recent post is one way, but when post is a year old, I really cant find it and I dont have time to something like that.
Well you could always order them by date updated.