Balls sliding rather than bouncing

I've made a 2D game in which a player is supposed to avoid balls bouncing around in a room. I've made a material for the balls and the walls which has no friction and the bounciness is set to 1, in hopes that the balls would continue to bounce around the room and off each other without losing speed. The problem is that if a ball flies into a wall at a shallow angle (if the velocity is anywhere near parallel to the wall), the ball will slide along the wall, as if it is stuck, rather than bouncing off the wall. What do I do?