Generally in the west, we tend to view China’s aerospace efforts somewhat dismissively. After some Chinese aircraft bear uncanny resemblances to designs from Airbus, Antonov or Sukhoi. Perhaps we might want to revisit that thinking. China’s Shaanxi Aircraft Corporation, part of the AVIC combine, has been building its Y-9 military freighter for some time. First flight was late 2010.
The Y-9 is thought to be a more advanced copy of the Antonov AN-12. The Y-9 has Chinese-made turboprop engines and a Chinese six bladed composite propeller. The table below shows that the Y-9 is not an exact replica of the AN-12, but it is close and larger. 
China feels good enough about its Y-9 to put it in competition for Thailand’s planned replacement of its C-130Hs. Lockheed Martin is offering the C-130J (although the J first flew in 1996, the core design is 1950’s era). In some respects the Y-9 offers more than the C-130J. Almost certainly the Y-9 will cheaper than the C-130J.
A win for the Y-9 will send an unsettling message.
Do you have an explanation why is China capable of building a good aircraft like the Y-9 or even build a space ship and at the same time there having so much problems with the ARJ21 or the C919?
Its still a newer version of the An 12 or its chinese copy the Y-8. So we cant really call it a new design and just follows in the Chinese tradition of ‘copy more’
Possibly the easiest explanation would be that the Y-9 benefited by being derived from the AN-12. While the ARJ-21 also was derived from a previous design, it is commercial and therefore more complicated. The C919 is an even more complicated project and is not obviously derived from the A320.