A very common bathroom plumbing problem is having a toilet that won’t flush properly or won’t flush at all. If the toilet doesn't flush at all, now matter how hard you pull the handle, then there could be a couple of things wrong with it, depending on the type and brand of toilet you have.
Concentric-float fill valve
The most common flushing mechanism used by professional plumbers nowadays is the more modern flush valve assembly that is all plastic with only a few moving parts and only one little circular rubber part controlling the flow of water to the tank, called a “Tank fill valve”. The newer concentric-float fill valve consists of a tower which is encircled by a plastic float assembly. Operation is otherwise the same as a side-float fill valve, even though the float position is somewhat different.
Side-float fill valve
The side-float design incorporates a float, usually ball-shaped, which is located to one side of the main valve tower at the end of a rod or arm. As the side-float rises, so does the side-float-arm. The arm is connected to a linkage which blocks the water flow into the toilet tank, and thus maintains a constant level in the tank.
Siphon flush valve
If the toilet uses a flush siphon, then the siphons diaphragm could be burst, the pressure inside the diaphragm is then drawn up into the chamber and escapes through the split in the diaphragm giving it little to no resistance. The more often you flush the toilet, the larger the cracks in the diaphragm get.
Although it is possible to replace the diaphragm valve, you may find it difficult to find a plumbing supply to sell you only the diaphragm. The shape and size of the diaphragm depends on the particular siphon model and manufacturer, often making it easier to replace the siphon instead.