When a certain number of replies are posted on a Message Blog or other thread it is added to another page and if you are unfamilar with how this works, might wonder what happened to your reply.
If you scroll down to the bottom of a post that is more than one page long you will see page selection options at bottom left and bottom right. On bottom left, you can click Next to go to next page, or click on the specific page #, or enter the the page number and go to it over on the lower right.
I have included a screen shot below pointing out the page selection area below. Since the Ning.com hosted site allows us to now post almost any type of content including pictures and videos, I'm sure this is done to prevent TOO MUCH data to be on one screen causing load times to be very slow. Once you get use to scrolling through the pages it is 2nd nature.
Here is that screen print: