You will notice that in your email delivery statistics there is a field under Audience Calculations that is for Total Filtered Out. This number can be confusing at times, below are the possible causes for your constituents being filtered out of a delivery.  Here are a few of the possible reasons members of your email audience may be filtered out of the group for receiving a message:

Group overlap- If the constituent is a member of multiple email recipient groups selected for the email, or if if they are in a target group and a do not target group they will be left out.
Already received the message- If the constituent has received the same email once before, Luminate will not deliver it to them a second time.
Opted out- Your constituent has opted themselves out of receiving email.
Interest opt out -Your constituent is a member of an interest group that opts them out of receiving this particular email.
Center opt out- Your constituent is opted out of receiving messages from your specific center.
Invalid Email- your constituent does not have a valid (BB750946) email address.

If you started a delivery, canceled that delivery and restarted it, then you might run in to an erroneous filter message "Has been sent this message already" (BB743722).