Why aren't my logged-in sessions sustained on certain forums?

I belong to a large list of auto enthusiast sites, and recently several of them have started required me to log in each time I access the site, even though I always check boxes like "Remember me each time I visit" when I log in.

It can't be a cookie problem. I have cookies that are several years old on this MacBook Pro, transferred over from my previous Powerbook G4, which were in turn transferred from my old Quicksilver desktop. I always keep cookies off until I need to log in to a site, and when I am firmly established there I turn the cookies off again. Its been working that way for me for years and until now I was never asked to log in - my sessions always persisted.

The problem started with a phpBB- based system about two months ago, after they upgraded their software. Then it spread to another phpBB forum.. Next, a vBulletin system started requiring log-ins each time I went there, and now its happening at another vBulletin site.

I thought it was a phpBB problem until it happened on those two vBulletin sites. Its nothing that I can't get around simply by logging back in, but I'd like my sessions to persist indefinitely like the scores of other sites I'm on.
What is going on here?