problem in displaying sensor output

Other Hardware Sensors
1

i am using optical speed sensor on my arduino uno to sense the rpm of a dc motor . the results is being displayed on i2c lcd , while the motor is functioning and the sensor is supposed to measure the desired rpm , the lcd shows 0 rpm but when the sensor is removed , the lcd displays the correct result.
here is my code:

int encoder_pin = 2; 
unsigned int rpm; 
volatile byte pulses; 
unsigned long timeold;
unsigned int pulsesperturn = 2;
#include <Wire.h>  
#include <LiquidCrystal_I2C.h> 

LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); 
void counter()
{
   pulses++;
}
void setup()
{
     lcd.begin(16,2);
   lcd.backlight();
   pinMode(encoder_pin, INPUT);
  
   attachInterrupt(0, counter, FALLING);
  
   pulses = 0;
   rpm = 0;
   timeold = 0;
}
void loop()
{
   if (millis() - timeold >= 1000) {
      detachInterrupt(0);
      rpm = (60 * 1000 / pulsesperturn )/ (millis() - timeold)* pulses;
      timeold = millis();
      pulses = 0;
      lcd.setCursor(0,0);
      lcd.print("RPM = ");
      lcd.println(rpm,DEC);
      attachInterrupt(0, counter, FALLING);
   }
}