A breakthrough for me.
Before this, my red bean will be slightly mushy after slow cook for just 1.5 hours. Until one day I bought the red bean from one of the shop near my house. I used to buy from Jusco. The red bean was still hard after 1.5 hours. So, I let it continue for another 1.5 hours. It turned out to be perfect. The red bean was very soft but still in good shape.
And, I received comment saying that it was the best red bean soup that I had ever cooked. I think it was the different type of red bean. I must go and buy more stock from that shop.