I've played with Xin a few times and everything seemed to be working just fine in that one.
Any chance you could consider making a properly formatted Bug Report post so that this may get addressed faster? It's worth noting however that these matters are much easier to resolve if you list any conclusions you've come to about why it isn't working.
Some possible reasons you may want to look into in a custom match:
Xin having too much/little attack speed
Xin having too much/little crit chance
It doesn't work every Nth time you use the ability
It won't work every N times you use it in a single life
It won't work if you're effected by X form of cc
It won't work on X champion/monster
If you use N attacks on one target, it won't work if you switch (details)
If you cancel N number of autos when it's active, it won't work
Best of luck