This is actually a fun one, and kinda has some parallels to building a nuclear weapon.
I tried this as a grownup because I finally managed to get my hands on saltpeter (could only dream of it when kid). Followed the instructions, mixed everything in correct ratios, lit it with great care and fanfare and... hiss fizzle. I was so disappointed! I think it came down to purity of ingredients and not enough surface area.
Point is, there are certain details of the process required to make it truly work, that are not readily known; in a similar way with nuclear energy, the theory is pretty well known but some nitty gritty details like the implosion or detonator design are not.
South africa was able to make a minimum viable weapon on a shoestring budget. They had access to nuclear reactors though.
As a kid I found saltpeter at an old-fashioned pharmacy and made gunpowder, and it also barely fizzled. I think you have to grind the ingredients much finer than a kid has patience for.