70728/2 Ninjago piece too short?
As this set is built on a plate-built base, it is quite possible that they are not pushed together tightly. I have ran into this numerous times when building the LEGO Modular Buildings as the second and third floors have a plate-built floor. Look through everything that has been built so far and make sure that the plates are pushed together all the way. LEGO does have some tolerance and viggle-room, but it is much less than a millimeter.