Actually, if you follow the references in the wiki article, you'll find that there was lots of documentation on how to make FOGBANK. The actual components were completely known, as well as the manufacturing process, however the specifics of the manufacturing process (-exactly- which machines) were lost. In fact, what you'll find is that when they finally got around to making FOGBANK (albeit at lower yields) the reason the original FOGBANK manufacturing process had such high yields was because the material they were using were actually more impure than the rerun.
FOGBANK is actually one of those materials that relies on impurities for it's qualities, and the original documentation did not note this (probably because they did not know), and it was only with more recent techniques that we were able to establish that.
Anyhow, the FOGBANK stuff is no where as simple as 'uh durr, silly engineers forgot how to make something'.