Syncing issues with Campaign vs CRM

Syncing issues with Campaign vs CRM

Hi, 

In my CRM I have a total count of 2348 clients, when I log into Campaigns I have a total count of 1311 so theres over 1000 people that I am not emailing.. My sync just consists of Name, Surname and Email.

Do you know why its not syncing properly?
or what could cause it not to bring all the contacts over?

What do I need to do?

Thanks
Andrew