Updated on 07/Dec
More pictures and I kind of knowing why. The last picture is when it's flushing. I found the water only comes out from the top of the rim, and there's no water comes out from the bottom of the rim. This makes the direction of the flushing water can't really PUSH the stuff into the pipe. I also have a look at other toilets, they behave the same. Maybe I worry in the wrong direction.
Updated on 06/Dec
I tried to unclog it by toilet auger. It worked a bit since I pulled out something dirty stuck in the pipe. I did it three times and also lubricate it with dishwasher and hot water. However, the flush is still weak. What I can reckon is, the water in the bowel will be replaced by the flush water. It's the stuff in the bowel didn't get flush down into the pipe. Might be an issue with the toilet itself? Maybe all I need to do is to change a new toilet?
Attached more pictures of my toilet bowl.
I put some toilet papers in the bowl and flush it. From the video, it shows the toilet papers just kept swirling in the bowl and there's no absorption happened to pull things into the pipe.
First, I'm sure the water level in the tanks has been set to maximum.
I also have tested this toilet by quickly pouring a big bucket of water to see whether the flush happens. Unfortunately, no matter how much water I pour, no flush happens. What I saw is a lot of water in the bowl and the water level quickly comes back to its origin. No absorption happened. Unlike the bucket test video I saw.
I then doubted might be a blocker in the pipe causing the flush is weak. I pour drain unblocker liquid into the bowl and flush it after waiting for a while. However, the flush is still very weak.
Actually, the plumbers came to here two weeks before and told me it's the tank cause the problem. They replaced the tank, but the problem still exists and they just disappeared and not answer my call. (It's another sad story)
£400 for the plumber and they didn't fix it. I don't want to be scammed again. I googled and studied in order to let myself understand the situation a bit more. But after trying all the possible solutions, nothing really works.
I came here for seeking advice and help. Any thought is very much welcome and appreciated. Thank you very much.