Paint Skin Weight tool doesnt apply

[Ni3ls]

Hello, I'm making a model but the rigging didnt go well in some parts. So I use the paint skin weight tool. I correct everything for some parts. But when I go to another bone and then go back to the edited one, the painting is done. I can start again then. How can I solve this?
Here is the example
1: I use the tool on the right leg.
2: I click on the left leg
3: I click on the right tool and nothing has changed