Because tutoring relationships are so strongly based on communication, sometimes things can go wrong. If you find yourself in this situation, it is important to address the issue quickly and calmly.
First, be sure of the facts. Overheard conversations, retellings by students or others all have the possibility for misinterpretation. Before getting upset, make sure you know what is true.
Second, calmly explain your concerns. Express your feelings in a way that focuses on your concerns rather than your tutor's actions. For example, "I feel concerns about your reliability, because you have cancelled the last three sessions we had scheduled," instead of, "You seem unreliable." This can prevent defensive attitudes on either side of the issue. If it is easier for you to address conflict through writing a letter, that is also acceptable. Be sure your letter is well organized and clear to prevent further misunderstandings.
Third, be prepared to offer suggestions for a solution, but not an ultimatum. "I'd like to re-schedule the sessions you've missed," instead of "If you cancel again, we're going to find someone else." Ultimatums essentially create power struggles, and won't be productive in resolving your situation.
The last thing to consider is the type of tutor you have. If, for example, your student is having trouble with an adult working in an after school program, you should address your concerns to the teacher in charge, or the school administrator. Likewise, if your student is attending sessions at a private tutoring center, you are better off addressing your concerns to the supervisor, rather than the tutor themselves. Of course, if you have hired a professional to come into your home to provide their services, you'll have to confront them personally with your issues.