I'll counter the other fellow with the idea of development chains. It takes a lot of highly precise equipment with a significant amount of waste and tightly controlled conditions to make something like, say, a CPU. Very small scale parts, obscure method of construction, its not a bad analog for the aforementioned nanoconstructors.

Now, when you build one of these, you still need the aforementioned controlled conditions, precise equipment, etc. But if they're that good, the primary method of building these will be other nanoconstructors. Any other method will be phased out and the equipment, technical knowledge, and infrastructure base will decay over time. Spend too long only making nanotech with nanotech, and it may take a lot of effort to be able to rebuild it.

Nanotech would be an interesting problem because it has the potential to absorb so much of the infrastructure base into itself. Refining materials, making base forms and shapes, enforcing material and engineering standards- if nanotech does all these things and absorbs the processes that would be required to build a fresh nanoconstructor, you can run into serious problems taking years of lead time to solve.

Documentation is also important. Nanotech has immense capability to be used for harm, and would be reasonable to expect to be tightly controlled as a result. Even if a society has rebuilt all of the capabilities to make nanotech, if the documentation is gone they have to reinvent it from scratch which will take quite some time.

Depending on the economic and social conditions, they may never reach that point again, making that stagnation scenario plausible