There's not enough information here to answer that question for sure. Some potential causes could be:
- You have communications limits on (which is the default for engagement programs) and the remainder of your records were over their limit for the day or week. This is probably the most common gotcha and based on the volume that didn't go out, probably the first place I'd look.
- You have leads that are not qualified to receive emails, such as unsubscribed, marketing suspended, blacklisted, invalid email, or empty email.
- You had a number of leads paused in the nurture rather than set to a cadence of normal.
- You have a filter on the smart list that sends the email that limits the leads (if you are using programs to send out your emails).
It could also be that some of your campaign members don't have email address, especially if you are pulling people from something like SFDC. I run into this issue a lot - where if I don't include an "Email Address IS NOT empty" filter in my Smart List, the number of qualified leads is exponentially higher.