A few things can cause this issue. The ball flapper in the tank is closing off to fast not allowing enough water to empty into the tank.
The ball flapper is not opening enough thus preventing the water from draining fast into the tank which creates the main flushing action.
An issue with the vent system. If the vent on the roof is plugged, perhaps a bird? it prevents the water from going down the drain properly sort of like creating a suction in the main drain preventing the water from clearing the bowl.
The tank is not filling high enough. If it shuts off to soon you may not have enough water to clear the bowl.
Have you tried using a plunger? If it was used and your not successful then look at the other ideas provided.
Keep us posted and we will see what else we can come up with. But those are the main reasons why the bowl does not clear the solids.
Some people complain that the newer 1.6-gallon toilets just never flush adequately, no matter what they do.