It's surprising that China has not been more forceful with NK. China's leadership are all closet capitalists and are very pragmatic. Every move NK makes only increases the size of the US military footprint in the region. Japan and SK are requesting the US to deploy missile defense batteries in the region. Missile defense systems that could very easily be used to degrade China's offensive and defensive missile forces. That is the last thing China wants to see.
I have been to both countries and talked with average citizens of both. The west vastly overestimates China's influence over the DPRK. North Korea might have been China's puppet 60 years ago, but that is not the case now. China is the DPRK's biggest trade partner, they know it, and they abuse that situation at every opportunity.
China has the longest land border of any country, and they border with 14 sovereign states (tied with Russia). They don't want any more adversaries on their doorstep, especially western-friendly ones. If Korea reunited, the north half of the country would probably become more and more democratic, and be open to the influence of the US. China does not want that. If North Korea's economy improved, they would be a low-cost competitor to Chinese industries. China does not have any incentive to act in the best interests of North Korea. They have a large interest in keeping North Korea a poor, isolated, backwater nation.
All of this creates a situation where China is intentionally holding North Korea down, trying to bully them around, and generally not being a good neighbor. After decades of this, why would North Korea take China seriously when it appears that China just wants to hold them back? China and North Korea are not best buddies like many people seem to think. At best, their relationship resembles the US-Mexico relationship, except that the border actually does have a fence, and China made North Korea build it.