Consider emailing students who received a low grade on an exam or homework assignment. Your email can encourage the student to make use of available resources. It can be helpful to express confidence in your student’s ability to learn to motivate them and decrease the stigma of poor performance. You can use mail merge to save time without the email appearing impersonal.
Example: "I noticed you didn't turn in the last homework assignment."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-encouraging-help-seeking