Rich:
A toilet bowl is nothing more than a glorified siphon.
With a siphon hose, you suck on the end of the hose to fill it with water (or any liquid) and as long as the outlet end of the hose is at a lower elevation than the level of liquid in the container you're siphoning from, the laws of physics take over and the siphon keeps going until something stops it.
It's exactly the same thing With a toilet bowl, 'cept you don't suck on the outlet end of the toilet.
Instead, the toilet tank dumps enough water into the toilet bowl fast enough that the volume of water overflowing the weir inside the toilet bowl and entering the discharge channel is sufficient to fill that discharge channel competely with water.
And, once that happens, them same laws of physics take over and turn that discharge channel into a powerful 2 inch diameter siphon hose, which sucks up the water in the toilet bowl.
Provided nothing interferes with that process, then you have Sir Isaac Newton's personal guarantee of a successful flush every time.
But, as Jadnashua says, there's lots of things that can interfere with the process:
- not enough water entering the bowl or not entering fast enough
- not enough room in your toilet's drain pipe to accomodate the water.
The way to tell if the problem is upstream or downstream of the water level in the toilet bowl is to pour a 5 gallon pail of water as quickly as you can (without spilling any water on the floor) and seeing if the bowl flushes properly then.
If it does, then the problem is upstream of the water level in the toilet bowl. Check that the tank flapper is opening fully, that the holes under the rim of the bowl are open and flowing, that the jet hole at the bottom of the bowl (if you have one) is open and flowing.
If it doesn't, then the problem is downstream of the water level in the toilet bowl. Maybe there's something caught in the toilet discharge channel, maybe the drain line from your house is already full of water and there's simply no room in that pipe for another 5 gallons.
So, do the 5 gallon pail test, and in my humble opinion, it's a real good idea to have the main drain line from your house cleared with a snake every 10 years as preventive maintenance.
If you can't remember the last time that main drain line was cleared with a snake, it's time to have it done.