The other side of the argument that might help you raise your current rates:
1. How long have you worked with this client?
2. If it has been months - at least 3 - send a note explaining that at the start of each year your rates are subject to a 10% (you decide the actual %) increase annually.
3. Discuss this with your client. Do not demand an increase.
4. Tell them you enjoy working with them and helping them reach their business goals, etc.
As Tonya wrote - this can be dicey. But worth a try. Timing is on your side.