They are all there, jumbled together, waiting for their time.
Each was a product of some incident, some random comment or an interesting news item. Of course, they were only partly formed. There is always something elusive about them, but at the same time any one of them could unexpectedly erupt when least expected. Their ability to come to the surface does not always happen at a time when they can be easily captured. It goes without saying that the advances in modern technology enables them to be caught when they do; ranging from electronic devices and back down the levels of sophistication to the humble pencil and paper.
Sometimes they lay dormant for decades. Where they are, they are safe. None of them are in a hurry to see the light of day. It seems that the few emerge from the many in their own good time.
No doubt that is why ideas for stories can be so elusive.