For the second part of your question, do the following steps :
1) Pick some of the random contacts of your segment to whom the email is not being delivered and see their individual contact profile to see they have not opted out.
2) Pull the soft/hard bounceback report from Insight for the email that you are sending (assuming you are sending the same set of emails to your target segment) and match it with the contacts who didn't get your email to see how many are bouncebacks
3) As a last step, look at the exclude list under setup to make sure none of the contacts are part of the exclude list. This could be one of the reason that email is not being sent.
Let me know what you find, if the problem persists then you need to double check all the campaign steps and everything to make sure all is setup correctly.