I have noticed that if I enter the forum and do not log in and look at 18th century for example, then log in I can not post in 18th century, all I do is come out of the forum and re enter remembering to log in problem solved.
That actually makes a lot of sense.
There are 2 systems at work here you see.
The forum stuff and the homepage/membership stuff.
They are separate entities and logging into one does not guarantee being logged into the other.
However the bridge is pretty good and I have never on any of my 3 accounts had an issue with being logged into one aspect of the site but not the other, however I do accept that it does happen.
Best thing is make sure you are logged in at the homepage and you should be good, clicking remember me should sort that though for you.