My problem is as discussed by other posts but I cannot resolve it. Fast trap waste will only dispose of enough water if centre plastic piece (with o ring seal) is backed off i.e. loosened. With everything tightened up the waste water will not empty quickly enough and shower tray would overflow. There is no problem with my waste piping, good fall directly to stack. If I loosen centre piece waste will run really fast and how it should be - please help?