It's usually a result of the flapper not being fully pulled up high enough so that it's buoyancy keeps it up while the tank drains.

It can be a few different things. The flush arm inside the tank could have displaced or bent downwards a bit so when you flush, the arm is not raising up high enough for the chain to pull the flapper into the vertical position. To repair, the arm can be reset to the proper angle where it joins the handle if it has slipped there. Depends on the design on how easy that is to do.

Sometimes the handle/arm junction is fine, and it's the arm itself that has simply bent downwards due to years of flushing. Most people just bend the arm it upwards a bit. Do be careful if you bend it upwards. Don't bend it so high that it now hits the tank lid when flushed. And make sure that the now bent upwards arm doesn't catch on the fill mechanism when it is actuated.

Sometimes leaving the arm as-is and simply shortening the chain can work. But it really depends on the arc-swing of the end of the arm.

Also, the flapper itself could be deformed. You flush, and the flapper deforms enough so it is just pulled off the seat for water to start draining from the tank, but due to the deformation, it doesn't get pulled high enough to overcome the suction from the tank draining. The outflow of water simply reseats the flapper when the flush handle is released. Replacing the flapper is best, but sometimes shortening the flush chain can fix that too.

Obviously, if you shorten the chain, make sure its still long enough so the flapper can reseat itself.

Mongoct, thank you very much.

I shortened the chain, but the toilet still flushed weakly. I will replace the flapper. According to the specification of my ultramax elongated G-Max toilet with sanagloss, the flapper model should be THU331S, which is temporarily out of stock in amazon. Amazon now has the THU175S model, which can also be used in the ultramax model, according to Toto technical support.

Don't forget, also make sure that the arm is still properly set and properly secure where the arm joins the flush handle.

It can sometimes become a little loose there, so the arm "droops". The arm ends up lower than it should be so when you do flush you don't get the full arc-swing that you need at the end of the arm to fully raise the flapper.

Hopefully that makes sense.

Mongoct, thank you very much. I checked and it looks the arm is properly set and secured.

I have 5 Toto toilets. Have had them for 12 years. Lately the one we use the most doesn't flush unless we hold the lever down an extra second or two. I found the problem is the lever - not the flapper or the chain. It's a different style. The lever doesn't turn the arm when you flush - the lever pulls the arm down on the outside of the tank which causes the inside to go up. There's a pocket on the inside of the lever for the end of the arm and that pocket wears through eventually. When that happens, it can't pull the arm down fully. I ordered a new lever and arm and I see it's been redesigned and that pocket has been reinforced. It was a bad or weak design to begin with apparently. I haven't found any other articles that discuss this problem. I wonder if some of the other brand knockoff replacements are also redesigned or only the new Toto levers.