XBee Shield Arduino External Power Issue

The XBee part of the entire thing works flawlessly no matter how things are being run.

Apparatus: Laptop, Arduino Uno, Sainsmart XBee Shield, USB XBee Explorer, 2 x XBee Series1 modules, Terminal Software

What we are doing: Send data through Terminal Software using XBee 1 on a USB explorer to remote XBee 2 which is connected to Arduino Uno using the Sainsmart Shield. Arduino sends a reply which we see in the terminal software. (Basically just using XBee to eliminate physical RX TX connections in a simple point to point communication)

Works perfectly fine when the remote Arduino + XBee are powered by a USB port of a PC (doesn't have to be the same PC. Tried with seperate PCs, one powering the transmitter and other the reciever.)

Issue: Does not work if Remote Arduino+XBee are being powered by (9V Battery(Brand New)/12V, 2A Adaptor/5V,2A mobile charger directly supplying through USB). BUT it starts working after pressing RESET (Sometimes, very rarely, have to press RESET 2 or 3 times but it does start working) and keeps working till the supply is not disconnected/turned off.

Important Note 1: As mentioned earlier, the remote XBee always works perfectly fine in all of the above cases as shorting RX/TX shows that the data perfectly and flawlessly echoes back.

Important Note 2: I don't think that the Arduino isn't starting up as to check that I am blinking pin13 LED which always starts working fine in any of the scenarios listed above.

Please help. Having to RESET every time after power up makes the whole thing pretty useless for any real world application.

Please help. Having to RESET every time after power up makes the whole thing pretty useless for any real world application.

We might need to see your code. My Arduino/XBee devices work fine using 4 AA batteries.

It was an issue with the XBee modules... Received 3 new modules and things started working perfectly with the new ones