Have you got the relevant joint selected in the option box? You can only paint on the areas affected by the joint once it is selected iirc. So if you want to paint the weights of back joint 3 you selected that to see what areas its is affecting already etc.

Randomly posting in the middle of the day as we have a plumbing problem and a collapsing ceiling thing, so I am taking my mind off the state of my house with a little browsing, lol.