The major "roadblock" is getting the nuclear materials - getting enough of the correct isotopes and avoiding the "wrong" isotopes.
Counterproliferation is a tireless and thankless profession. The term for technologies that can be used for good or evil are generally called "dual use". One of the common ones is HF. Hydrogen Fluoride can be used in the uranium enrichment process as well as in refining gasoline/petrol. There have been no new oil refineries built in the US in the past half century, yet more gasoline, and higher octane fuel, has been produced (in the US) while closing some refineries. HF is used in a large number of industrial processes, I merely listed 2 which Iran would love to have. Their gasoline refineries are working with 1960s level technology, which burns up most of the crude oil they extract. Supplying them with HF would let them modernize their refineries to the point where they could export far more crude. Or let them make "yellowcake" for refining into highly enriched uranium. I'm aware that no sample of enriched uranium collected by IAEA shows enrichment past what is needed for their domestic nuclear reactors (about 4% enriched for power reactors and 20% for the one that makes radioactive chemotherapy stuff).
The Nuclear Non-Proliferation Treaty [0] says that signatories of the treaty are forbidden from transferring dual-use materials & technologies to countries that have not signed the treaty. This was why a Bush-era trade deal with India [1] was so controversial. Neither India (nor Pakistan, nor Israel) have signed the NPT.
Counter-proliferation is an active process. You have to actively block pathways to building nuclear weapons, and constantly run intelligence operations tracking these.
The difficulty in making a nuclear weapon is in making a clean, effective, efficient, and high powered one. Making one with a relatively low yield is really not especially difficult. The principal difficulty is merely obtaining the fissile material.
Two hemispheres of accurately machined fissile material, a vacuum pump, some really solid steel tubing, and some TNT will make a bomb.
The average engineering shop doesn't have what it takes to safely machine fissile material. It's actually quite hard - unless you don't care about irradiating and poisoning your workers, and possibly risking a squib explosion.
I understood that hundreds of explosives experts were involved in designing the shaped charge that created the compressed plutonium critical mass.
Restraining proliferation was simply impossible, if the bar was this low.