Marketing Mass Emails, can only email once

I have created a marketing campaign, which has leads and members applied to it. 

I then created a mass email for that campaign and it sent the email as expected.

If I create a new mass email for the same campaign the program then does not allow me to send the new email to same members.

If I add new members to the campaign then create a new mass email, the program emails only the new members.

Any ideas why the program does this or best way to resolve without having to create a new campaign.