There's no reason to think that people are able to remember everything they think about. We are constantly going through internal ruminations, whether we are paying attention to them or not. I don't think that the internal monologue is something that we turn on and off at will or consciously. I'm not sure if the stream of consciousness that is verbal is always running but when it is running our focus on it fluctuates. Sometimes one can really focus on observing the random thoughts that happen inside the mind. Sometimes one can direct the stream and try to focus where it goes. Sometimes one is completely unable to grasp the internal messages.