Strange toilet issue

Try these
Make sure your water is up to mark on tank
Make sure 1/4″ tube is in overflow pipe
Make sure your flapper stays up for complete flush
Make sure the flushing rimg holes are cleaned out use wire Make sure hole in bottom of bowl is open
Make sure you donot have a partly stoppage in bowl are pipe.coat hanger for that

Also try a 5 gallon bucket of water and dump into bowl; if this water goes down with no problems, then the problem is limited to the operation of the toilet itself.

Thanks for your quick response, I will try it first.

There are 2 more things I have noticed that is not the same as before.

1 The water level stay in the bowl is lower after flushing.

2 There is a strange sound after flushing. I think it happened when the sewer gas enter.

Sounds like a vent problem, is 1/4″ tube inside 1″ overflow pipe thats where the water comes to bring it up to level.

I have checked all the things you mention except 1/4″ tubes inside 1″. What is 1/4″tubes inside 1″ and where is that? Thanks.

I have done a test with a cigarette butt.
When I flush, the water goes into the bowl, the cigarette butt will only swirl on top of the water. When the water level become low then the cigarette butt will enter the passageway but it comes back.

Where your water comes into tank is called a ballcock, on top ofit there is a 1/4″ tube that loops over and goes in the 1″ overflow tube to refill your toilet bowl to make a trap seal. If it is not in the 1″ tube and you flush the toilet the bowl does not fill to make trap seal and then when you flush part of the water fills up trap seal so there is not a enough water to make it flush. Go to http://www.google.com and ask in search the insde of toilet tank.

I just check that and it seems ok. I think I need to call a plumber. Anyway, Thanks for your help.