USceneComponent's son'son can not show in blueprint component searcher!

I have a class A: public UsceneComponent,
and class B: public A.
I open a blue print, I can add A to my actor, but I can not find B.
how to do? thankyou

Oh, My God, I find I miss
UCLASS( ClassGroup=(Custom), meta=(BlueprintSpawnableComponent) )