OnValidate() not called Unity 5.1.0f3

OnValidate() is only called upon script load or change from edit to play mode. Why?
Not working in 5.3.5 neither. When i change values in my script, OnValidated should be called, but it’s not.

In my case the problem was, that I used a custom editor. When you use a custom editor, the OnValid() function is not called anymore automatically. Thus, you need to call it e.g., when a parameter changed or in the OnInspectorGUI() function, if it’s no matter if it’s called more often.

Just to be clear, the problem is that it’s not being called during editor mode right? Because if the problem is that it’s not being called in runtime, then that’s because it’s not supposed to be called then.