Some students are very good at hiding the facts. Take away all distractions during their study time. Insist on performance first - rewards come second. Buy your student a Daily Planner and help them learn how to effectively use it. Time management and organization of tasks are critical for success.
Parents don t be afraid of the work.
Insist your student seek tutoring from teachers, tutors on campus, or outside agencies. It is easier to complete something if they know why they are doing it. It might help to post these goals on the bathroom mirror as a reminder each morning. When one only does the minimum, one can hardly expect to achieve the maximum. Grades are not the only piece of the puzzle.