[W]hen I sign in it doesn't let me post.
Can't start a new topic
Can't post if signed in
Can't vote in polls
Membership has it's privileges.
I'm using the Safari browser supplied on my iPhone, so I presume all iPhone users can't play on your site, whether signed in or not. GL growing your site like that.
There's no reason why the forum sortware should behave like that. Can you tell what error message it shows when it doesn't let you post?