So we may recall a dream we had the night before (understanding in the right brain of images) - proceed to congradulate ourselves at having done so - only to return to the memory and find it GONE! Why does this happen? Because the follow-on images from the image we recalled have no corresponding words associated with them. So the conscious mind looks for images based on conscious logic. Not dream logic/wordless logic. It fails and we can not remember any more.
Further, things can happen in dreams at an enormous rate of speed. Sometimes we have a dream and the knowledge filters down through the subconscious in days or weeks or even months (then we have deja vu because we have no clue how we 'we have been there before.') From Thought Mechanics I show that when a pattern match is found with the pattern-matching algorithm holographically comes up with a match - 'understanding' takes place. It 'dawns' on us what a thing is. That is why deja vu is this experience. We understand something. We 're - cognize' something. Unfortunately, the thing we understand happened long ago and the conscious mind can no longer make the association/connection. When we see a building or something, for whatever reason, this triggers a dream-logic image sequence. Understanding happens and we are perplexed consciously and mistake the understanding that happens with the building as one of having been there before.