Player is unable to join campaign

Sorry, this might be a newb question, but I've looked around for a question like this for awhile and couldn't find an answer. One of my players can't seem to join the campaign, however four others have. There is still room in the campaign for more players, so I'm thinking it could just be lag. I invited the player, and he's pressed 'accept' yet he says it's not appearing on his campaign list, and apparently he's unable to join. There is no error message. So far we've tried restarting the browser, and relogging into Roll20. Help would be most appreciated :) The campaign link is <a href="https://app.roll20.net/lfg/listing/26623/chapter-1-the-broken-blade-courier" rel="nofollow">https://app.roll20.net/lfg/listing/26623/chapter-1-the-broken-blade-courier</a> and the player in question is "The King".

I assume you sent the player the join link from your campaign? Do you have the option to invite them via email? That uses a different link and sometimes seems to work when the join link doesn't.

Thanks Aaron! I forgot about the join link, I'm kind of new to DMing. He can join now. What I did was press the 'invite to campaign!' button next to their usernames.