hi recently installed a central shower drain with a removable trap plastic that has a rubber seal at the top of the trap, as soon as the trap is put into the drain it blocks compleatly with the trap removed the water flows as fast as you would like, when the trap is installed its blocked solid when the rubber seal around the top of the trap is removed and the trap put in without it on it all works perfactly is there any way of putting the trap in with the rubber seal in place that i am not doing properly