Touch Shield and Serial communication

I am using the 2.8 tft touch screen shield on my arduino mega, and by itself it works great. However, im trying to read inputs from an arduino UNO connected to mega’s serial1, rx and tx. Whenever the screen is not plugged in, Mega reads UNO’s outputs but as soon as i put the shield on the Mega, communication stops. If i restart the Serial monitor, random characters are displayed. Also, the LED on pin 13 of the uno starts blinking rapidly when the shield is on the Mega, but otherwise, it stays off.

Is the shield preventing the mega’s Serial1 from communicating? or is it something else?
either way, how can I solve this issue?

TFT touch Shield use digital pin 2~13 and analog pin 0~3.
And it not use Arduino Mega Serial Port 1 for communication.