Does your toilet have a very slow or "lazy" flush? When a toilet that once flushed properly begins to flush poorly, slowly, or not at all, the problem is often a clogged drain--so consider this first. If other toilets or drains are not working properly, then the drain is no doubt the issue. See How to Fix a Toilet Clog.
If other drains seem to be working fine, check the problem toilet by dumping about 1 gallon of water into it from a bucket. If it flushes down fine, the drain is probably okay. If it flushes poorly, the drain is probably clogged (see above).
When you flush a toilet, clean water in the tank goes rushing down the flush valve to wash the bowl and provide enough water pressure to force the waste down the drain. If your toilet has a lazy flush but the drain seems to be clear, poor movement of water from the tank to the bowl is likely to be the problem.
Remove the tank's lid and see if the water level inside the tank rises to within 1/2 inch of the top of the overflow tube--the tall hollow tube that is connected to the flush valve. If the water level doesn't, there may not be enough water to provide sufficient force for a full flushing action.
To raise the water level, bend the float arm (see How a Toilet Works, Toilet Plumbing Diagrams) or manually adjust the refill valve. Allow the water level to rise to within 1/2 inch of the top of the overflow tube. If someone has added water-saving devices to the tank such as bottles or dams, remove those devices to allow more water into the tank.
Next, determine whether water may be leaking from the tank into the bowl. If it is, this could diminish the flow into the bowl when the toilet is flushed. Use a pencil to mark the water level inside the tank, then turn off the supply valve behind the toilet and wait a couple of hours. Then check your mark to see if the water level is lower. If it is, please see How to Repair a Running Toilet.
While the tank is empty, test the flapper to be sure it hinges smoothly and moves up and down properly. If it is a little balky, smear a little petroleum jelly on the ears where the flapper connects to the flush valve. Make sure the flush handle, when pressed, lifts the flapper all of the way open so that all of the tank's water can rush into the bowl before the flapper flops back down onto the flush valve. With most toilets, the chain between the flapper and the trip lever should be relatively taut; if it is slack, disconnect and shorten the chain. Just be sure it isn't so short that the flapper can't seal against the flush valve when closed.
Also check the handle and the trip lever. If they wiggle or have too much play, they might not allow the flapper to raise all of the way or they may cause the flapper to misalign with the flush valve. If they are loose, try tightening the nut or spud. If necessary, replace the handle mechanism.
The next thing to check is the toilet's siphoning action. When you flush a toilet, the water that rushes into the bowl from the tank moves through passages around the bowl's rim and through a chamber in the front of the bowl. Some of it rinses the bowl through small rinse holes along the underside of the rim. The movement of all this water into the bowl creates pressure that forces the waste out through the drain at the bottom of the toilet. The jet hole, a small hole near the bottom drain, provides the necessary suction for completely emptying the bowl.
Turn the water supply back on and allow the tank to fill. Use a mirror to watch the water cascade down around the inner rim when you flush the toilet. Water should flow evenly through all of the rinse holes. If it doesn't, mineral deposits from hard water may be clogging some of the passages. This problem is most common in regions that have very hard water.
Scrub the bowl with a toilet brush, especially up under the rim where the rinse holes are. Turn off the water supply valve and flush the toilet to empty the bowl, and then plunge the bowl to force out most of the remaining water. Wearing rubber gloves, use the mirror and a bent coat hanger to clear the rinse holes and the jet hole, being careful not to scratch the porcelain.
If necessary, you can use lime remover such as CLR to dissolve mineral build-up, but this will put your toilet out of commission for about 8 hours. Pack the rinse holes with wet paper towels held in place with plumber's putty and, holding the flapper open with one hand, pour a bottle of lime remover into the overflow tube and flush valve. Let the liquid sit for about 8 hours. Then, wearing rubber gloves, remove the packing and putty. When finished, turn the water supply back on and try flushing again.