I've read accounts that after the Romans left, much of the architectural knowledge left with them, as the Britons couldn't build like the Romans. Why was this knowledge never oassed onto them? Was there nobody that could reverse-engineer the process?
Hello, this question's cropped up a couple of times recently (not that that should discourage you asking!) and I answered it most recently here.
The tl:dr of that is that it's broadly a question of logistics, not technology. Without the Imperial trade and supply networks and with the retreating Roman army apparently having taken most of the engineers and architects with it, the Romano-British simply lacked the resources to continue to build effectively in stone, or eventually to even do basic maintenance. They do surprisingly well for some time but eventually simply have too many threats on too many fronts to deal with.