Perhap change the material, the light, the geometry. Really there are a number of things that can fix it, but you need to give more info on the scene for any of our advice to be effective. Please supply more information and we can help you a whole lot more. Thankyou for cooperating, and I hope we can fix your issue soon enough. Although with what you have given now the most we can do is throw guesses and ideas at you about what is wrong.