Recently I came to know about the problem of comment box in Blogger. Actually the problem is that when someone open the page and he is unable to comment on that posts although your settings are right.I tried many codes but it didn’t work for me Finally I came to know that the main problem is that we have Reader Comments off for the post.
>there select your blog
>click posts and comment
>check that your comment setting is right or not
>check that you have selected Embedded comments
If you have all settings correct then open the post editor and check on right side whether you have allowed users to comment upon the post of your blog.
This small change helped me to solve my comment box problem, Hope so it also solved your problem.