There are a lot of variables missing out of this equation..Form being what comes to mind first..are your shoulders blades pulled together, is there an arch in your lower back and driving with your legs to further lock your shoulders/back into the bench. When you unrack the weight are your elbows flared or tucked, are you squeezing the bar or trying to pull it apart to involve the triceps, the location of the bar when you lower it, and are you pushing in a straight up motion or arching the bar back towards the rack..sorry for the rant but to me technique should be addressed