I think that most of the research is happening behind our backs and they are making progress there. Perhaps if fans move to a new location, they can see something new?
I think there are a lot of behind the scene development, testing and validation work between what it takes to make a prototype fly reasonably well, work all the control surfaces in the air, basic weapon carriage tests on the one hand, and development of dedicated mission equipment on the other. There would naturally be a lull in outwardly visible progress between the one and the other. Right now j-20 is probably in that lull.
During f-22 development, the fly able prototype was unveiled in around 1990. This corresponds to 2010 on j-20 timeline. The prototypes flew for about 3 years, ending around 1993. This would correspond to 2013 in the timeline for the j-20. Then all outwardly visible progress on the f-22 stopped. No additional fliable prototypes appeared for 5 years, while the design was refined, requirements updated, and detailed development of mission equipment proceeded. For f-22 the fruit of 5 years of nonflying development wasn't seen until 1998, when the 3rd fliable airframe joined the development. This would correspond to 2018 for the j-20 timeline. Then 18 more fliable development airframes joined the f-22 program in the next 4 years.
F-22 program occurred when USAF was oversized and drawing down its strength at the end of Cold War, and the F-22's role and future was undergoing review. So f-22 program wasn't being pushed as fast as it could have been. China might conceivably push j-20 program fasterr than f-22 program's actual pace. But even if j-20 program followed the original pace for f-22 program as scheduled during the Cold War, there still would have been a lull between completion of prototype tests and start of developmental airframe tests.