A couple of the toilets on my place simply don’t flush hard enough, often it takes a couple of flushes to do the job.
What factors how hard/fast a flush a toilet produces? Is it mainly the amount of water in the tank?
The factors would be the design of the toilet, the amount of water in the tank, and the opening of the flap. In some cases, the water level can be adjusted upward. If the flap doesn’t open fully or quickly enough, that can be fixed. I don’t believe there’s anything else feasible to do.
Another factor is a slow waste pipe. Pour an extra gallon of water into the bowl while it’s flushing and if the contents still don’t flush away, I think it’s most likely a slow waste pipe.
Some people used to put a brick in the cistern as a simple way of reducing the amount of water used in each flush - check for this and remove any such object.
You may be able to increase the volume of water in the cistern by adjusting the float valve to let the cistern fill to a higher level before shutting of the inlet valve. If it is a plasic design (most likely) there should be some sort of screw to adjust the level. If the float is on a metal rod, just bend the rod.
As the flushing mechanism wears out, the flush can become less and less effective. I think it is not so much the amount of water as the flow rate that is affected. You may have to replace the flap valve or even the entire flushing mechanism - they don’t last forever. If you have a number of similar toilets and some are working and some are not this is probably the answer.
There are all sorts of little things you can do to try and fix the situation, believe me, I tried. But when it comes down to it, you should consider installing a pressure-assist toilet.
My toilet has performed flawlessly since I posted that thread above. I love the fact that your ears pop every time you flush it. 
Open the top of the tank & adjust the float so that it allows more water in the tank to create a bigger flush. Be sure to check a line that should be in the tank to show you how much water you can keep in it so that you don’t put more water than that in it.
I discovered this by accident. Recently the float valve went out in one of my toliets. Standard cheap plastic ball cock type. Replaced it with an all brass ball cock one. Big Difference in flushing. Less clogging, etc.
Could be that it sends more water down the overflow pipe, could somehow be caused by the fact that the refill tube empties directly (and rapidly) at the bottom of the tank.
Worth a try for less than $15.
The newer johns are designed with water conservation in mind. The tank only hold about a gallon and a half. The only way to get a clean flushing toilet these days is a pressure assisted version.
Pressure flush or search the junkyards for an older toilet. The newer 1.6 gal models are notoriously inefficient flushers but required by federal law in new construction.
I’ve got one of these too. No secret as to what you’re doing, but that stuff is GONE!
The latest issue of COnsumers Report has a write up and rating on toilets. Just incase you’re considering replacement.
They’ve improved toilets since the 1.6 gallon models were mandated. I installed an Eljer Patriot in my folk’s house and it flushes as good as any high flow toilet.
Thanks for all info folks.
“The only way to get a clean flushing toilet these days is a pressure assisted version.”
Not so. I’ve got a new, low flow, quiet Toto and it’s very effective without pressure assist. It also has a lid that quietly lowers itself.
However, if your toilet used to work better than it does now or if you’ve got an old house it may be worthwile to get someone out to clean out the waste line from the house to the street. That can greatly improve the function of all the toilets in the house.