As far as I know, you get one gravatar per account. An account is identified by the email. One WordPress account can have multiple blogs, but they will all share one email and thus one gravatar.
Gravatar may have accepted your different pic for another email, but when you are commenting while logged in with your WordPress account, it will use the pic associated with your wp.com/email account.
I have multiple blogs on wp.com, associated with multiple accounts (emails) and depending on which I'm signed into that is the gravatar which appears.
I have never tried a gravatar account associated with only an email only to comment anywhere.
So yes, I am saying that you need to separate your WordPress accounts in order to have different gravatars when you comment.
I'll modlook this thread for staff to look at. They may have different information about how to do what you want.