I rarely check my gmail spam folder, and was surprised to see quite a few forum notification emails in there. Not sure what Gmail's algorithm is because I did get a few notifications in my Inbox.
Anyway, it might not be a bad idea to check your Spam folder, and if you see those, mark them "Not Spam" so hopefully Gmail will not be too confused in the future, although there is no guarantee there and it's out of anyone's control.