Bold titles are simply used to show which threads you haven't read yet. The reason your threads are not bold is because you've already read them because you created the post and you also view them after creating them.

Your posts appear bold to me and anyone else who hasn't read them. Now when I view this topic again it'll be un-bolded but if someone posts after me it'll be bold again until I click to read it.

This is just how vBulletin (WF's forum software) handles things.

Just wait till you ask how to get These > ▌< characters in your title!