Problem of BluetoothChat example

Last year (June 26, 2015) I have a post to show how to "Import Android code sample of BluetoothChat to Android Studio". It work perfectly AT THAT TIME. But I re-try it again on update Android Studio 2.1, and test on ASUS Zenfont 2 running Android 5.0 and XiaoMi RedMi 2 running Android 4.4.4; it almost NOT WORK - no problem in pairing and connecting, but fail to send/receive data: sometimes one-way chat, sometimes totally cannot chat!!!

Then I change the Project Structure to target sdk version Android 4.4 (KitKat), it can be improved, but not 100%.


I have no idea is it related to new Android Studio/SDK? or related to new Android version? or related to individual device(s)?