This is a subject I've discussed in several past answers (which I will link in a moment), but to begin with I need to stress that 'a society as ancient and advanced as Imperial China' is, to put it bluntly, a meaningless phrase. All societies are theoretically equally 'ancient', and 'advancement' is its own can of arbitrarily-defined, meaningless worms. We may as well ask how a society as 'ancient and advanced' as Rome fell to the Turks in 1453. Presenting imperial China as an 'ancient' society also massively flattens the real changes that occurred in state and society over the over two millennia between the foundation of the first imperial state in 221 BCE and the end of the last imperial state(s) in the 1910s. (See this answer for a bit more on that.)

On the question of specifically military developments, Tonio Andrade's The Gunpowder Age (2016) is far from the final word on the topic, but it is one of the more comprehensive ones. One of the major ideas Andrade puts forward is that the locus of military technological innovation shifted back and forth between Europe and East Asia in the medieval and Early Modern periods: gunpowder weapons were first developed in China, then were brought to Europe by the Mongols in the 14th century, European iterations on these weapons were then brought to Asia in the 16th century, where Asian states iterated on these imports, and many were arguably ahead of European states in some respects until the end of the 17th century. Some polities maintained parity well into the 18th century (Indian states especially); some fell behind by the middle of the 17th (Japan particularly). Andrade puts the Qing divergence in terms of field weapons as starting mainly around 1750; I personally date it more to around 1700. However, he also notes that East Asian polities lagged considerably further behind in naval and fortifications technology – two very important things for the Opium Wars, where the Qing was being attacked (so some better fortifications would have been nice) by sea (for which a more competitive navy would have been nice). Discussion of those are their own, separate issue which I don't feel fully qualified to comment on.

But as for development on land, I discuss some of the issues here and to an extent here. One issue I could have discussed in the first answer is finance, but that comes through in the second.