Nothing about producing nuclear weapons, let alone miniaturized thermo-nuclear devices optimized to get the most efficient reaction during detonation is cheap. And the knowledge wasn't necessarily lost, that is impossible to determine since we're dealing with highly classified secrets regarding the construction of a nuclear weapon.

It's possible that while making fogbank they merely got a contaminated batch of boron. Let's say, for example, you find your grandma's recipe for apple pie. But when your mother tastes it she says it's a little off. You then eventually realize that you used apples that weren't ripe yet, or that grandma used apples that were beginning to get soft. Thats not really the same as "lost knowledge" it's more an experience gap.

Clickbait always makes these things sound like we've lost this information and now we're screwed because it can never be relearned and no one knows anything oh no doom and gloom. Truth is lots of knowledge, through-out history, has been "lost" and then "rediscovered" primarily because this info becomes less valued/important and eventually someone needs it again so they rediscover it. Happened alot with mathematical theories and concepts, like calculus.

No one wants to lose knowledge but no one wants to pay to keep it alive the proper way (master/apprentice system that ensures these tricks of the trade are continually passed on) so we favor a system where specialized experts (scientists) are called in to solve these issues and rediscover the tricks of the trade using the scientific process. Either way you pay for the knowledge but we wanted to save money and shutter a bunch of cold-war era defense companies when the cold-war ended. The consequences of that are when you want to resume making nuclear weapons of the quality America demands you have to hire those scientists to figure out all the pesky details like fogbank.