1) Demographics. First the one child policy then the total collapse of the birth rate even after the one child policy was lifted. China is already older on average than the US. It is the fastest aging society (among the major economies) in the world. Their dependency ratio is growing very fast.
2) The largest real estate speculation/debt bubble in history.....their dwarfs the size of the one in the West which caused the 2008 global financial crisis.
3) the only way to try to solve problem #2 above is to try to lean even more heavily into manufacturing and exporting their massive surplus production. But the 2 largest markets - the US and increasingly the EU now - are steadily turning more protectionist and will not allow them to dump into their markets. There are no other untapped markets anywhere near the size of those two so any suggestion that they could just export more to Africa/South America/India, etc automatically fail.
Biggest problem? Tyranny
Overview of Free Speech in China
Free speech in China is severely restricted. The government exercises strict control over all major channels of information, including the press, internet, and public discourse. This control is enforced through censorship and the suppression of dissenting views.