eNewsletter not sending out to all my members

As the Topic Subject says:

I get the following message when I choose the checkbox:
"Don't send to people who've already received this: "

"All members have already received it or no user is subscribed!"

When I chose the whole group, without checking the box, the email get's sent.
But my last send only went to half the subscribers, 136 out of 267, and I obviously don't want to send a double email to those people who already received it.

What do I do?
Thanks
Haroun