Container Cannot Access Host Machine


I have host A with a few docker containers, for example, container A1 with port mapped to 9000;
and I have host B with a few containers, for example, container B1 with port mapped to 8000;

On container A1:
I can access container B1 with http:// B:8000 ;
I can ping host machine A;
However, I cannot access host machine A, for example, http:// A:9000

On container B1:
I can access container A1 with http:// A:9000;
I can ping host B;
However, I cannot access host B, http:// B:8000.

Where I did wrong?

anyone? thanks so much!