Knocking sound at the front when brake is applied

I have been coming back and forth to the dealer because of this knocking sound at the front when the brakes are applied. When I drive my vehicle early in the morning or it has been parked a while, no knocking sound is being heard. But after half an hour or more driving, I started to hear this knocking sound. The dealer overhauled it and even changed the slider pins because it was not moving freely. It was okay on the first week, but then the knocking sound was back. What must be the problem? I can only hear the noise after driving half an hour more. Thank you very much and waiting for your most appreciated reply.