North Korea, and the nations it trades missile technology with,  are trying to develop technology for producing solid fuel rockets. Japanese investigators have revealed that a Japanese firm illegally sold such equipment to Iran and Japan. Solid fuel rocket motors are tricky to build, as the "fuel" is actually a slow burning explosive. For use in a ballistic missile, the explosive material must be manufactured to exacting standards, and then poured and solidified under precise conditions. One mistake, and your missile will either be way off course, or explode on launch. This is why so many nations have stayed with the simpler (to build) liquid fuel rockets for so long. Even the Russians had a hard time developing solid fuel rocket motors, not really getting it right until the 1980s. And it's not just a matter of technology, but of precise manufacturing methods.