Do the rest of the fixtures in the bathroom drain well? If they do not drain well, my guess would be the venting of the Plumbing system. If all of the other fixtures in the bathroom drain well, then there is probably something in the trapway of the toilet. Assuming it is just the toilet that is having problems, then a closet auger might do the trick. Hope it helps.
The main drain is plugged. You may need to remove the toilet and snake directly down the drain.
The toilet itself is plugged or the pipe from the toilet to the drain is plugged. It may be that the tub and sink have their own drain pipe that may connect to the main away from the toilet drain. Plunge the toilet or sometimes if you fill a 5 gallon bucket with water and pour it into the toilet as fast as it will take it, that will flush out the line. It is a greater amount of water all at once an forces out what the clog is.
either the vert is plugged and there is no air getting through or there is no vent at all
Home toilets do not use "suction power" to flush. The inertia of the water falling from the tank when you flush pushes water through the trap and down the drain. If there is any significant amount of blockage in the drain, the water will not be able to move at enough velocity to get past the trap. In other words, if the drain is blocked, water can't flow fast enough to flush. You'll need to get the drain cleaned out to make it flush.
pull the toilet. Make sure the trap is clear on the toilet, and then snake the toilet line.
The toilet and the sink both drain into the grey water waste lines. Because they are connected by piping, sound can travel through the pipes from the toilet to the sink.
You would need to use a container of water like a 5 gallon bucket and pour it into the toilet. As you pour it in, the excess water will cause the toilet to drain.
You should get your drain lines snaked
If the toilet works each time on one flush, they use roughly half the water of a standard toilet. You would then use half as much a month on the toilet. They do not always flush completely on the first flush and the toilet is only a part of the water bill.
A water cistern is the part of a toilet that holds water which is then used to flush the toilet.
The sewer line is plugged or collasped. The sink and bathtub may drain because they drain at a slower rate. Toilet is all at once and the drain can't handle it. Water has to go somewhere so it backs up in the sink and tub.
Toilet tank