We are talking about the WS-15 engines. Initially, the aircraft with them were supposed to become part of the Air Force in the first half of 2021. However, the inspections showed technological problems, so the deliveries were postponed to the fourth quarter of the year. A few days ago, the Chinese media reported that again everything was wrong with the components for the J-20. The journalists concluded that the WS-15 engines were never brought to readiness.
One of the known problems of power plants is the loss of thrust of the engine when it reaches operating temperatures.