Material not showing for leaves

Development Rendering
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Hi, I’ve a problem since this morning and I can’t find any solutions. I looked here for solutions, but nothing fixed it.

Here it is : I’ve made a Tree with Blender, following this tutorial : Unreal Engine 4 Speed Design - Creating Stylised Trees in Blender for UE4 - YouTube
and I came up with this :

250494-1.png
250494-1.png1600×900 208 KB

Then, I imported my mesh into unreal engine.
I made a foliage material :

250495-2.jpg
250495-2.jpg1600×900 296 KB

But, when I try to put my leaves material that I created on the leaves of the static mesh, It just disappears :

250496-3.jpg
250496-3.jpg1600×900 367 KB

I don’t understand why. Could someone help me please ? It makes this with all the tress you can see in the background. Moreover, doing this method for the grass (for example) work just fine !

Thanks a lot