Analog to Digital Conversion on ECG via Arduino

Hello everyone,
For my graduation project, i am using Arduino UNO and i need help.
I am using Arduino in order to make analog to digital conversion on ECG signal. I get ECG signal by using an ECG circuit and the output of the circuit is connected to the A0 pin on the Arduino. Also, Arduino is grounded. However, most of the time (95%) i get no signal, serial plotter shows just a straight line as signal. The problem is not related with the ECG circuit itself because i have tried also AD8232 ECG module and the result was again no signal.
I tried another Arduino UNO, i tried Arduino Mega 2560, i changed the connection cable, i even changed the subject that ECG signal coming from.
I cannot find out the reason of the problem. I need suggestions, please help me.

So you can see both your code and schematics and cannot figure out what the problem is, but you expect someone else to figure it out, without seeing either?

I think the problem is most likely somewhere within the circuit, the wiring itself or the code, none of which you gave any information about. It would also help if you give more details on what happens in the remaining 5% of the time.

I also wonder why you think the lack of output signal is not related to the circuit, using the argument that the ouptut (no signal) didn't change upon changing sensors.

we are not fore-tellers,

not when you haven't posted the code ( use tags please ) and schematic

wvmarle:
I think the problem is most likely somewhere within the circuit, the wiring itself or the code, none of which you gave any information about. It would also help if you give more details on what happens in the remaining 5% of the time.

I also wonder why you think the lack of output signal is not related to the circuit, using the argument that the ouptut (no signal) didn't change upon changing sensors.

Thank you first of all. I will try to give more detail about the problem.
In order to obtain the ECG signal, I am using INA114 instrumentation amplifier and a 47ohm resistor to set the gain to 1000. Resistor is connectd between the pin-1 and pin-8. Pin-2 is connected to right arm, pin-3 is connected to left arm, pin-4 for V- and pin-7 for V+. Pin-5 is grounded. I also ground the right leg of the subject. When I connect the output of the instrumentation amplifier (pin-6) I can see the peaks of the ECG very well, the circuit is okey.
Then, I am connecting the output of the ins. amp. (pin-6) to the A0 pin of the Arduino UNO and ground pin of the Arduino to the ground. I am using 5V from the power supply in order to fed the ins. amp. Here is the code that I have used to do Analog to Digital Conversion:

void setup() {
// initialize serial communication at 9600 bits per second:
Serial.begin(9600);
}

// the loop routine runs over and over againa forever:
void loop() {
// read the input on analog pin 0:
int sensorValue = analogRead(A0);
// print out the value you read:
Serial.println(sensorValue);
delay(10); // delay in between reads for stability
}

when we use AD8232 ECG monitoring sensor instead of ECG circuit, the code:

void setup() {
Serial.begin(9600);
pinMode(10, INPUT); // Setup for leads off detection LO +
pinMode(11, INPUT); // Setup for leads off detection LO -

}

void loop() {
if((digitalRead(10) == 1)||(digitalRead(11) == 1)){
Serial.println('!');
}
else{
Serial.println(analogRead(A0));
}
delay(1);
}

In the remaining 5% part, although I follow the same procedures, Arduino takes the real time ECG that I must obtain in the other 95%.