How long have you been waiting?
When I first sent out the emails, I thought that I could get all the
responses in a day, but only one person who used to be nice did that, and I
didn't hear back from anyone else. I was quite frustrated.
However, somebody who responded after 8 days turned out to be a nice person
who wrote a nice letter, although he was always slow in response.
I often follow up once a week. If they don't respond after two or three
emails, then I move on.
Also, you can contact all the authors, not necessarily the first authors.
But for the same paper, I often email one at a time.
Keep up your spirit, and you will get independent reference letters, many
many good ones!
