Until very recently, stagnation or very slow progress were pretty much the norm for human societies. Feudal Japan, ancient China, the Middle Ages in Europe--centuries would pass with little or no technological development whatsoever. There are a number of reasons for that: humans are reflexively suspicious of change, the humans in charge are actively hostile to change, and it takes a long period of political stability, economic abundance, and societal tolerance in order to sustain ongoing technological development.
Look at the technology that mankind has already lost, or nearly lost, and had to reinvent over the years. The Greeks had geared mechanisms, enormous land and sea vehicles, and metal-framed structures, and that knowledge is pretty much gone. The Chinese had Admiral Zheng's fleet of preposterously large ships and basically decided that they weren't even worth the effort to record in detail (which is weird for the Chinese, who pretty much invented bureaucracy). We're still trying to figure out how the Vikings rigged and navigated their ships. And so on.
There are any number of reasons why the inventor/engineer class of people might become so few in number as to be barely able to maintain existing technology let alone invent new things.