The Romans, for example, were excellent at recognizing and adopting technology and ideas other civilizations had. However, just about the only actual invention they made was concrete. After Rome fell, there was a dark-age period where much of the knowledge that that the Romans had gathered was lost. Then there was a period of extremely slow growth as a few new agricultural practices were implemented and some of the lost classical knowledge was rediscovered due to the crusades. But if you were a landed baron, you would expect the same yield in rents and crops from your land each year, not a constant rise in productivity. Money changed hands, but there wasn't much in the way of economic growth.

Except technology continued to improve during the dark ages. For example, during the late Roman Empire waterwheels where rare toys the philosophers/mathematicians occasionally played with, by the end of the dark ages, there was one on almost every suitable site in Europe. And thanks to lack of written records, we don't even know how this happened.

On the other hand, the quantity of production did in fact greatly decline with the collapse of the Roman Empire.