I recommend making a smart list with the following filters to help you determine what went wrong.
1. Member of Program (it sounds like you used the email program to send it?)
2. Unsubscribed = false
3. Black Listed = false
4. Marketing Suspended = false
5. Email Invalid = false
6. Email Address "is not empty"
That should help you identify everyone who should have qualified, with a couple of caveats. If a lead was sent an email but it was blocked as spam in the last 24 hours, it won't send. Also, if duplicate records (they have the same email address) qualified, it will only send an email to one of them.
I especially recommend adding in those filters above one at a time, so you can see how each category contributed to the email not sending.