Are the walls up and tiled yet? If not, take the pan out and use some sand mix mortar under the pan to ensure all of the nooks and crannies are filled and supported. Mix it kind of wet and smush the pan down so it is level. If the walls are up, you may have to live with it. The use of the mortar mix under the pan is not optional if you want a good result (no creaks, deflection), in my limited experience. My unprofessional opinion.