I had 2 new Kohler Devonshire toilets installed this summer. When solid waste is flushed, sometimes flecks wash back up. I was told by the tech services person that we have blocked air vents. It's hard to believe that two separate vents are blocked in the same way. I think it's the poor flushing action of the toilets (class 3 flush).

It is posible that you need to adjust the water level in the commode tank. Either add more water or make it less. Adjust this level until it flushes best.