Hi Teresa! Email Deliverability can be kind of nebulous...there are so many different factors that go into whether or not you make it to the inbox, and that can also vary by ESP. If Marketo receives a response from the recipient's email server, it will log an "Email Delivered" activity. If it doesn't, it's hard to know whether or not it was actually delivered.
How long are you waiting after an email send to view this activity? Sometimes emails can take a while longer to be delivered on the recipient's side if they have different server settings or spam filters. Sometimes an email will soft bounce and be held in pending for several hours or even days before it is delivered.
Are you looking at the Email Performance report as well? That would indicate if some emails are being held as pending.
Also look for commonalities in the domains of people who don't have a "Email Delivered" log. Are they all from the same domain or group of domains?