As a music teacher, I'm always trying to tap in to my student's intrinsic motivations. But sometimes, those intrinsic motivators are not well developed...yet. In those instances, I have to use extrinsic motivators (i.e. prizes, competitions, awards, certificates, grab bags, etc).
For the majority of my students, I require a certain number of "good" practices per week. ("Good" practices are those that are conducted with a respectful attitude and appropriate amount of effort and time. The parents must sign off that it was indeed a "good" practice.) If the set number of practices is reached, then the students may pick a prize bag, and take it home with them. Its amazing how excited the young musicians are when they go get to pick out their bag and take it home with them.
If your musician's teacher does not provide that form of extrinsic motivation, then you as the parent can definitely provide that. In my house, with my own kids, I have a candy dish on the piano. After the kids put forth a good practice, they may pick one piece of candy. They tried to practice all day long at first, trying to get many pieces of candy! I did have to limit it to one piece of candy per day, but it has definitely been helpful in getting them going.
You know your child well, are they being adequately motivated?