Basically, it’s shaped weird and won’t make a seal. It’s a WaterSense toilet that flushes very efficiently with 1.28 gallons, with an unusual configuration of input/output under the water. Almost like a channel from front(ish) to back. If you try to use a plunger like those pictured, part of the channel isn’t covered, so you just push water back out into the bowl. Good thing I was trying it with a clean new toilet! The wide deep beehive shape lays rubber all into the space, pushing the water down into the exit hole.