If you have manually added students to your course but they don't show up in the QuickMail list of potential recipients, the students have probably been added to the course without a role. To fix the problem, go to the Gear Icon (top right of page) → More → Users tab → Enrolled Users. Find a student that is not showing up and verify that the "Roles" column is empty for that student. Click on the person with a plus symbol icon on the right side of the column (it is hard to see) and add the appropriate role. You should now be able to email the student using QuickMail.