A coding problem or improper interrupt usage?

Question

As I should expect(?), the PIR on pin#2 should trigger the trigsensor state at any time the board is running. However, I may get one trigger then nothing else. I have a GSM shield on top of a UNO. I know that I still have some tweaking to do, but I'm trying to solve one issue at a time. Thanks for any insight. The other functions seem to perform correctly. When I enter an incorrect code, the alarms operates properly. When the interrupt is triggered, the alarm response should be the same.

///////////////////////// initialize & includes //////////////////////////
#include "SIM900.h"
#include "sms.h"
#include "Keypad.h"
#include <GSM.h>
SMSGSM sms;

boolean started = true;
int PIR_SensorPin = 2;
int Alarm_OutPin = 11; //green
int Light_OutPin = 12; //red
int alarm_count;
boolean volatile trigsensor = false;
const byte interruptPin = 2;
int z = 0;
int i = 0;

//////////////////////////// setup keyPad //////////////////////////
const byte ROWS = 4; // four rows
const byte COLS = 4; // four columns
char keys[ROWS][COLS] =
{
  {
    '1', '2', '3', 'A'
  }
  ,
  {
    '4', '5', '6', 'B'
  }
  ,
  {
    '7', '8', '9', 'C'
  }
  ,
  {
    '*', '0', '#', 'D'
  }
};
byte rowPins[ROWS] = {
  6, 7, A2, A3 // row pin# on keypad to Arduino pin#, ie Row Pin #1 goes to Arduino Pin #6, etc.
}; // connect to the row pinouts of the keypad
byte colPins[COLS] = {
  3, A0, A1, A4
}; // connect to the column pinouts of the keypad
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS );
const char PIN[5] = {
  '9', '6', '7', '9', '#'
}; // PIN number
char key_input[5] = {
  0, 0, 0, 0, 0
}; // used for comparison

//////////////////////////////// setup ///////////////////////////////////
void setup() {
  Serial.begin (9600);  // Establish Serial connection.
  pinMode (Alarm_OutPin, OUTPUT);   // Set pinMode.
  pinMode (Light_OutPin, OUTPUT);   // Set pinMode.
  pinMode (PIR_SensorPin, INPUT);   // Set pinMode.
  digitalWrite (Alarm_OutPin, LOW); // Set output pins to LOW for start.
  digitalWrite (Light_OutPin, LOW); // Set output pins to LOW for start.
  digitalWrite (PIR_SensorPin, LOW);
  Serial.println ("GSM Shield testing."); // Serial message that GSM shield is starting up
  if (gsm.begin(4800)) {    // Set GSM shield to recommended 4800 baud rate.
    Serial.println ("Status = READY");
    digitalWrite (Alarm_OutPin, HIGH);
    // PIR as interrupt
    attachInterrupt (digitalPinToInterrupt (2), trigger, RISING);
    //attachInterrupt ((0), trigger, RISING);
  }
  //  started = false; //////////////////////////////////////////////////////////////
  Serial.println ("System Ready, at the keypad loop!");
}

//////////////////////////////// void loop ///////////////////////////////////
void loop() {
  readKeypad();
}

////////////////////////////// void trigger ///////////////////////////////////
void trigger() {
  trigsensor = true;
  started = true;
}

//////////////////////////////// void alarm ///////////////////////////////////
void alarm() {
  Serial.println (started); Serial.println (trigsensor);
  //  digitalWrite (trigsensor, LOW);
  Serial.println("at Alarm");
  // while ((timedelay - time) > 0); Serial.println (time);  time++;
  if ((started == true) && (trigsensor == true)) {
    // (sms.SendSMS("**********", "***Motion Detected in Studio!***"));
    Serial.println ("MOTION detected: SMS Sent");
    for (alarm_count = 0; alarm_count < 5; alarm_count++) { // Cycle outputs if triggered
      digitalWrite (Alarm_OutPin, HIGH); delay (2000); // On/Off/On/Off/Off
      digitalWrite (Alarm_OutPin, LOW);
      digitalWrite (Light_OutPin, HIGH); delay (2000);
      digitalWrite (Light_OutPin, LOW); // End by turning off light
    }
    digitalWrite (Alarm_OutPin, HIGH);
  }
  else { }
  readKeypad();
}

//////////////////////////////// read Keypad ///////////////////////////////////
void readKeypad() {
  char key = keypad.getKey();
  if (key != NO_KEY) { // if a keypad input
    key_input[z] = key;
    Serial.print("Key_input["); Serial.print(z); Serial.print("] = "); Serial.println(key);
    z++;
    if (z >= sizeof(key_input)) {
      z = 0;
    }
    switch (key) {
      case '*': // resets key_inputs to "0"
        z = 0;
        break;
      case '#': // pressed to "enter"
        z = 0;
        for ( i = 0;   i < 4 ;  i++ ) {
          Serial.print(key_input[i]);
        }
        Serial.println ("-------");
        delay(100); // for extra de-bounce
        checkPIN();
        break;
    }
  }
}

//////////////////////////////// check PIN ///////////////////////////////////
void checkPIN() {
  Serial.println ("@ checkPIN ");
  int correct = 0;
  int i;
  for ( i = 0;   i < (sizeof(PIN) - 1) ;  i++ ) {
    if (key_input[i] == PIN[i]) {
      correct++;
    }
  }
  if (correct >= sizeof(PIN) - 1) {
    Serial.print ("# correct = "); Serial.println (correct); Serial.println (sizeof(PIN) - 1);
    correctPIN();
  }
  else {
    incorrectPIN();
  }
}

//////////////////////////////// correct PIN ///////////////////////////////////
void correctPIN() {              // do this if correct PIN entered
  Serial.println ("@ correct PIN ");
  started = false;
  digitalWrite (Alarm_OutPin, HIGH);
  readKeypad();
}

///////////////////////////// incorrect PIN ///////////////////////////////////
void incorrectPIN() {           // do this if incorrect PIN entered
  Serial.println ("@ incorrect PIN ");
  started = true;
  // add time delay before alarm, via keypad??
  alarm();
}

Answer 1

None of the code you posted ever sets trigsensor to false, besides the initial value.