I have a toilet off of my kitchen that has worked fine for a decade. Recently, it has been draining slowly, and sometimes not at all. When it does not drain immediately it fills to the rim and then drains slowly. It is not vented to the outside; instead, there is a screw vent/filter in the basement. Three, different plumbers have tested the system and when they unscrew the filter and flush the toilet, it flushes fine. They have replaced the filter with a larger one and that has not helped. The other toilets in my house flush fine. All, roof vents have been checked and are clear. The plumbers have run a snake from the toilet to the drain line in the basement and all is clear. They have advised that I do not need to run an augur from the drain line to the street because if there was a blockage there would be water backup elsewhere, which there isn't. The clay drain line was replaced with PVC piping around 20 years ago. The three plumbers advise that I vent the toilet to the outside. None of them can figure out why the toilet worked for a decade and now does not work well or at all.