If its because of engines than anything is plausible. Information of ws-15s development is limited and as of now, mid- 2015, prototypes have yet to be tested with it. If its service date is 2018 than there's 3.5 years left for developing and testing. Whether it's deemed a short window is up to the policy-makers to decide if the date needs to be pushed back or not.
hopefully at least al31m1, if not al31m2, which should be available by then. but there's also no reason why domestic ws10 variant should not be available, the much rumored 140-ish kn variant of ws10 that is reportedly ready by today.
Im not sure if anyone knows of this, but there are some rumours going around on Facebook defence communities that this Chinese magazine known as "Aerospace Knowledge" is reporting that the entry of the J-20 into PLAAF service has been pushed from 2018 to 2020 due to issues with the engine. However its only one particular source thats reporting this.
I don't see the point of implementing one stop-gag measure when the other is readily available. Unless WS-15 isn't ready by mid-2020s, there is no point to equip WS-10s.