I know many bloggers in the blogger world having been having issues with people being able to post comments. I just read a post I thought I would share that states, newer accounts don't seem to be having the issue but some older accounts would not let people leave comments. If this is happening to your blog, this article states you should change your comments to pop up window display and it should make it to where you don't have to log in again to leave a comment.
Just thought I would share. I know my blog was having this issue, not sure if it has been resolved but I went ahead and switched to pop up comment window just in case!