I've recently had several people with Gmail accounts inform me that they have been unable to leave comments on my blog. I've been having the same problem and know how irritating it can be. So I decided to google it and found some information along the same lines as my problem.
This is what I found.
If you're having a problem a little like this:
You finish typing your message and hit enter only to have this message pop up: "your current blog account does not allow you to leave a comment..." ( or something along these lines)
Then try this:
When this message pops up, go ahead and sign out. Then log in again with my same account only this time unchecking the "Stay Signed In" box.
I tried this out and was able to leave as many comments as I wanted. I then told one of my friends and she was able to leave messages too!
Hopefully this helps you out! I look forward to hearing from you, if I haven't already!