Shapekeys issues

[Guh~hey~hey~♫]

Lately, I came across a weird issues with shapekeys. Be it shapekeys that were already on the model or shapekeys that I created in blender, they work perfectly in blender, work perfectly in unity too, but once in game nothing happen.
What's weirder is that it's inconsistent. Some of my model have complexe shapekeys (like having the entire outfit vanish from the model when the key is used to 100%) and they work perfectly all the time in game, while other have very simple ones (for blushing or pulling out their tongue) that refuse to work.
Then sometimes they do work but, for some reason, no longer work later on....

I'm at loss if anybody got a solution for this ? Is it just that thing about mirrors not properly rendering some things ? Or is there some sort of limitations for shapekeys ?