While there are several potential issues that could cause a toilet to not flush completely, one of the most common is low water level. The float could be set too low, or the flapper valve could allow the water from the tank to leak to the bowl. Plugged bowel inlets also reduce flush efficiency, according to Bullseye Plumbing.
The float valve controls the amount of water that flows through the toilet with each flush. Most toilets have a mark inside the tank that designates the minimum level for proper flushing. Adjusting the flow below this mark reduces the head of the water so the trap does not make a full siphon to empty the bowl. If concerned about the amount of water used with each flush, consider adding a brick to the bottom of the tank to reduce the volume while maintaining the height of the column of water.
The flapper valve allows water into the bowl from the tank. It closes to allow water to fill the tank. A worn flapper valve does not seal correctly and allows water to leak below the designated level before the float causes the toilet to refill. Replacing the valve solves the issue.
Evaporation causes the buildup of scale inside the toilet bowl and the tank. This scale sometimes blocks the inlets that allow water from the tank into the bowl for the flush. Clean the scale from the inlets so the toilet flushes correctly.