Android subscriber can get msg but "rostopic echo" cannot?

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Hi, everyone. I am new to ROS Android and I'm playing with android_tutorial_pubsub. I run the program on AVD and run roscore on Ubuntu Desktop. As we know, the program on AVD creates a publisher as well as a subscriber, both on topic /chatter. The publisher publishes message "Helloworld n" per second and the subscriber recieves messages and show in TextView.
On my AVD,the TextView shows "Helloworld n" correctly. And on my Ubuntu Desktop, when I excute rostopic list, I can see a topic named /chatter. And the rqt_graph seems correct, too. But when I excute rostopic echo chatter (or /chatter), I can not get messages "Helloworld n". When I see rqt_graph, it shows that both android node and rostopic node subscribe to topic /chatter.
So here comes the problem. Why does rostopic echo get nothing but the android node can get messages while they're subscribing to exactly one same topic? I'd appriciate it if anyone can help.