After nearly 2 years of service, the kitchen sink waste decided to dump everything into the cupboard.    The elbow piece was connected to the short leg of the u-bend. Except when I tried to refit it, I found the clear plastic ring was not pliable rubbery material but hard plastic and did not fit over the diameter of the elbow. And there is no rim or feature on the elbow to clamp everything right when I use the threaded part to tighten the joint.   what gives?