Cactus don't bloom!

[XP_2600]

I have Astrophytum Asterias grafted on Selinicereus, last summer its produced something neither flower bud nor offset , its remains small and green (you still can see it in the picture) this year its started a flower bud for the first time but it fall before blooming , any idea about the reason / solution? thanks.



This is the incomplete bud..