Arduino Mega 2560 R3 & Wireless Shield v2 not working

Hi all,

I am working on a robot that can be controlled by joystick and voice recognition wirelessly. Everything works as expected until I change the arduino board on the receiving end.

Working:

Receiver - Arduino Uno R3, XBee Series 2 module on Arduino Wireless Shield v2 that is stacked on top of Arduino Uno
Transmitter - Arduino Mega 2560 R3, EasyVR Arduino Shield stacked on Mega 2560, XBee Series 2 module on Arduino Wireless Shield v2 stacked on EasyVR Shield

Not Working:

Receiver - Arduino Mega 2560 R3, XBee Series 2 module on Arduino Wireless Shield v2 that is stacked on top of Mega 2560
Transmitter - Arduino Mega 2560 R3, EasyVR Arduino Shield stacked on Mega 2560, XBee Series 2 module on Arduino Wireless Shield v2 stacked on EasyVR Shield

I have run out of digital pins to use for an LCD display that I am incorporating and hence need to change to Mega 2560.

So far, I have reached at a conclusion that the Mega boards are transmitting properly as I can see that from the LEDs on the Wireless Shield but the problem is that they are not receiving properly. I have tried interchanging the Mega boards but that has not helped. As soon as I put the Arduino Uno back everything works normal.

Any help regarding this issue will be appreciated. Thank you in advance for your time and support.

Links:

Arduino: http://arduino.cc/en/Main/arduinoBoardMega2560
Wireless Shield: http://www.digikey.com/product-detail/en/A000064/1050-1023-ND/2784004
EasyVR Shield: http://www.veear.eu/products/easyvr-arduino-shield/