If a toilet does not appear to have enough suction to fully empty the bowl, you will waste water by having to flush the toilet one or two additional times. For the right suction, or siphon action, to be created for the bowl to fully flush, the correct amount of water must be delivered to the bowl at the right speed. The toilet itself must also be clear of obstructions.
Insufficient Tank Water Level
The fill valve inside the toilet's tank is responsible for filling the tank with fresh water after each flush. The fill valve shuts off when that water level reaches a predetermined level, which is the amount of water that is needed by the specific toilet to create a full flush. If this level is incorrectly set on the fill valve, or the fill valve is faulty, the tank will not supply the necessary amount of water to the bowl with one flush.
Even if the tank's fill valve is working properly and allowing in the correct volume of water, the right amount may not be allowed to enter the bowl. If a flapper does not rise and stay up long enough to allow all the water that is supposed to drain into the bowl to do so, a full flush won't be possible. Check that the chain connecting the flapper to the toilet handle's trip lever is not too long, which would not allow it to rise sufficiently.
Blocked Rim Holes
Unblocked rim holes are one reason why ongoing toilet cleaning and maintenance is vital. Left uncleaned over a period of time, rim holes may develop blockages caused by hard water deposits in the toilet water. The rim holes are the series of holes located directly underneath the inside rim of the bowl. Hold a small mirror inside the bowl, and you can see them in the mirror's reflection. Because the siphon action that flushes a toilet bowl depends on both water volume and velocity, this action is impeded by the slow flow of water coming through blocked rim holes.
A toilet drain that is only partially clogged will not create the usual amount of suction in each flush, and the toilet will flush slower than normal each time. Your first remedy against a clog is to use a toilet plunger, which differs from a sink plunger by having a fold-out flap on the bottom of the plunger's cup. Use the plunger on the toilet drain, making a series of 12 down and up movements with the plunger. If unsuccessful at removing the clog after two or three series of attempts, use a closet auger, which snakes through the drain to reach deeper clogs.