Don't work harder - work differently
It's very common that we tell our kids they need to work harder when grades are not where they need to be. And it's very possible that they do need to put in more hours and effort. However, too often, the message of "work harder" is received by teenagers as "keep doing what you're doing...but do it better!" It shouldn't be a surprise, then, that doing more of the same things gets more of the same results.
The message I advocate, instead, is "work differently." This emphasizes the need to change the overall approach and routine to schoolwork, and is less daunting for students who already feel that they are "doing the best they can." This may seem like a subtle nuance, but I've found it to be effective in my students.