Hello, I have run into this problem before and its really annoying because i cant figure it out. So basically registered users can post comments on pages, I made a fake account to test this and when a users presses on reply or post a comment they can write the comment but when they click post a comment there is no response. Like you can click it as many times as you want and nothing will happen.
Here are the options I have set for commenting: http://tinypic.com/r/2h84gf6/6
If you want to test it yourself you can go to revpvp.com and register
Something else to know is that i have Mingle Forums plugin