The Chinese engine manufacturing problems cost us J-10.
The J-10 were to be purchased by PAF starting in 2003. But with the clause that all such jets should have a Chinese engine.
The Chinese failed miserably in manufacturing a home grown engine.
Only in the last two three years they have matured the technology.
And now they are first meeting domestic needs and export will come after.
Also the fact that although they have matured the engine design, the manufacturing techniques are still slow and turnover is very low.
Thats why Pakistan avoided WS-13 at the moment as China cannot provide enough engines quickly.
However JF-17 is designed for being able to swap components and being able to use various manufacturers.
If later the Chinese can maintain a reliable supply line of jet engines, then Thunders can be fitted with Chinese engines.
I doubt there was any clause saying the J-10 should come with a Chinese engine. Or else you wouldn't have accepted the JF-17 without a Chinese engine either, the threat is after all the same. In fact, with China's access to Russian tech for the AL-31, the J-10 would have been a far safer option than the RD-93, which China doesn't operate. So this talk of engine being the reason is a red herring.
The real reason is actually very simple. With the exception of some performance related advantages, there is no difference in capability between JF-17 and J-10. The weapons carrying capacity is pretty much the same, avionics are also pretty much the same. In fact, I would say that in the Indo-Pak setting, the JF-17 is a superior candidate over the J-10 due to its slight advantage in range performance and the PAF's current experience with the F-16, which has a similar design philosophy as the JF-17, other than the fact that the JF-17 is produced in Pakistan to a significant extent therefore making it easier to upgrade compared to the all-import J-10, with minimal access to its technologies.
And when it came to the performance related advantages, it would only reflect in WVR combat, but that's pointless since even our Mig-21s come with HMDS and high off-bore sight missiles, making it irrelevant. So why would you pay a lot more money to gain no capability and also inherit all the problems associated with a new design? The JF-17 came out of a family of proven designs in the F-7, so it was logical to continue with it rather than get stuck with a brand new design with no control over it. Not to mention the Chinese would have downgraded everything anyway.
As for the WS-13, as long as the PLAAF doesn't operate it, then the Chinese are not going to fix any design related problem. They will just ignore any problems that are difficult to solve and you will get stuck with a lemon for a long time.