Lack of literacy is also a problem. If there is no practical way of sharing how to do your wonderful invention with other great thinkers, no one will be able to replicate it after you die. Each inventor has to start from scratch, as it were.

Then there's also social upheaval. While many significant advances have been made due to war, if the society is so traumatized by the conflict that the people basically revert to an earlier stage, all the new advances are discarded and forgotten in favor of mere personal survival.

After the fall of the Roman Empire, folks basically forgot how to make roads and aqueducts of that quality for what, a thousand years or more?