It’s not about how much material you have, it’s about how you detonate it. Serious yield (thermonuclear, megaton bombs) is achieved through “blow back”, you use the initial blast to get an even more critical mass on the second stage. And that, it turns out, is really hard to do.
We don't know to this day whether NoKorea has successfully detonated a Plutonium bomb. We know they are refining and stockpiling the metal, but a number of blasts observed in North Korea going back several years have all been estimated at sub-kiloton range [they've been detected via listening devices because they haven't even been powerful enough to tweak the seismometers -- at least not the civilian ones.]
Some weapons experts speculated at the time that those tests weren't even "kind of successful" low-yield. It's actually possible that they got themselves blown up improperly handling the high explosives needed for the implosion, and the results seen weren't nuclear explosions at all.