Discussion – allow comments

I am using twentyeleven 3.3.2 wp.

I have two sites running with the above install. I have recently noticed a difference and don’t know what setting I have missed.

On one site, under the posting window, is a box called Discussion. You check or uncheck:
– Allow comments.
– Allow trackbacks and pingbacks on this page.

On the other site, it is not there at all. I have checked all of the native settings – General – Writing – Reading – Discussion etc, and all are identical.

Is there another area that I should be looking at? It’s seems quite strange for this to happen even with different plugins.