Most of the time after thinking extensively about a problem which I would like to come up with a solution for, my brain gets tired and refuses to listen orders anymore. This is when it starts to go off tangent and deviates to ponder on other subjects.
Therefore in, I realize it is most productive for me to be to be working multiple problems of very diverse nature at the same time. That way the brain is can be kept interested on the subject it is working on.