Chinese couples can't afford a second child, no matter what Beijing wants
https://edition-m.cnn.com/2019/02/12/asia/china-second-child-economy-intl/index.html?r=https%3A%2F%2Fedition.cnn.com%2F
As a country becomes rich, many people take it for granted that they buy a lot of surplus things. So, it means it will cost much more money to raise one person after the country becomes rich.
As many developed countries have the similar cases, it will also be difficult for China to solve low fertility rate.
