WordPress Pages “allow comments” meta option can't be checked

I'm trying to allow comments on WordPress pages, but when I check the page's "Allow Comments" meta checkbox option ( http://ift.tt/1vktD1V ) and save page, it does NOT save settings, it returns to an UNCHECKED box again! Don't know why?!

My default discussion settings as the following: http://ift.tt/1CPzgm0

PS: it works well on POSTS, but PAGES does not!

Any ideas about how to fix that?

Thanks in advance!