Hi Conner,
Yes, assuming that you ticked the "remember me" box, then the issue is a cookies issue.
The site will send you a cookie when you login, and your PC is supposed to send it back with every request it makes - be that a page request or a forum post.
If your PC doesn't send the cookie, then the forum will assume you are a guest, because it doesn't know who you are.
If you use Internet Explorer, have a look at the Tools/Internet options menu on the Privacy tab. Mine works well set to Medium. Alternatively, have a look at your firewall settings, as some of the paranoid ones block cookies. You could add
http://www.keratoconus-group.org.uk to an exclusion list from where cookies are allowed.
Best of luck!