Problem Flushing Toilet

[SingleCoils]

So my toilet has been giving me a problem flushing now for a few months. It does not matter if it's full or empty it just wont flush. Maybe one time out of 15 times. It does not overflow at all and usually the water in the bowl just swirls around.
I have tried plunging and it helps but it's not the typical plunging reaction where you plunge a few times and it goes town. It takes a lot. I have tried dishsoap. I have tried multiple overnight vinegar and baking sodas. And like I say it does not overflow and the water does not rise high in the bowl. Usually just swirls around.
So here is what I figured out and it works every time. Right after I push the lever to flush it I instantly pour one cup of water into the bowl so it hits right where the hole is and presto. It will flush everytime.
Would anybody have any ideas on what is wrong? Two more toilets in the house and all flush perfectly everytime. Thanks for any help.

 

[Breplum]

If you can raise the level in the tank, then try that.
might be something lodged in the trapway.
We would normally pull the toilet if using a good toilet auger doesn't solve it.
Also, as toilets age, they build up mineral surface in the trapway (out of sight) causing long term poor flush.

 

[Sponsor]

 

[Reach4]

have tried multiple overnight vinegar and baking sodas.

No baking soda. The baking soda neutralizes the vinegar, and just generates carbon dioxide gas. Vinegar by itself could be useful. It is a weak acid. You likely should try a stronger acid such as phosphoric.

 

[John Gayewski]

Pull the toilet inspect the trap way.

 

[Eman85]

I had a toilet that did the same. After plunging and snaking and cleaning all of the holes under the rim it still wouldn't empty. I was able to raise the water level in the tank, took some rigging, and it would flush every time. The permanent fix was to install it in a dumpster and installed a new toilet.

 

[Peterson]

It sounds like the toilet could have been cast wrong when it was being made, therefore not creating a vortex/suction when flushing normally. I'd replace it.