What's wrong with my sakura tree?

Hi, guys! This is my first time to ask questions here.
I made a Japanese Sakura tree in Speedtree. Then I import this tree in UE4 .
It looks like this:


Here are the material and shader model pictures.



But when I put it on a level, there are some strange dark spots on the flowers.



I checked the material and the model even flipped and fixed the model’s normal,
the DARK SPOTS are still there! I have no idea to solve this, can somebody know how to do it? Thanks!