I dunno.
The NorKs built their nukes from drawings that they bought, and not from any home-grown effort. It is known in intelligence circles that the blueprints that they got (from A.Q.Khan) were an old Chinese design that was doctored to the point that any device built from them would fail.
The first test proved this out. Note that no country in history has ever had a nuke test fail.
This means that they would need several tests just to work out the bugs that are built in to the plans, much less miniaturize it. The last test was a brute force effort to make it work, and the efficiency and reliability are suspect at best.
Constructing a test device in a cave is a long...LONG way from installing a device on a missile and delivering it accurately, reliably, and successfully to a target.