The public claim is that the quality control and reliability on the WS-10 production lines is poor, but there's evidence that there's more to it than just that. The WS-10 is produced mostly natively, but its design is largely reverse engineered from the AL-31F. In fact, its turbine blades are imported from Russia. This corroborates reports that the Chinese lack the design expertise at present to design a fighter engine from scratch.

Also, keep in mind that the WS-10A is derived from the AL-31F, which was first flown on the Su-27, but has since been replaced in Russia by modernized AL-31FM1 and 2 or the new 117 family engines. The FM and 117 series engines have generally better performance than the older F/FN parts, and likely are more efficient. Thus, we can see that the Chinese are working off an old design.