Speed control loop for line following Robot

I am working to control the speed in my line following Robot (using arduino uno),by using proportional constant Kp in a closed loop, in my code I calculate the RPM from The Right Motor and the Left Motor,I used Timer2 interrupt every 50ms to calculate the speed (Rpm) from these Encoders.

I used Timer 0 and Timer 1 to generate pwm signals with a VARIETY duty-cycle=75 % to control the tow motors speed,
I measured The RPM for the Left Motor by using Tachometer it is 100 RPM,
I changed the Kp constant But my Encoder Output still Give me about 80 RPM (no change).

so my question is :
is there any problem in my code or the way I do the controlling loop or understanding the controlling loop.

This is the Code :

//encoders variable
volatile bool counting;
volatile unsigned long events0;//the number of pulses from Left Motor in 50 ms;
volatile unsigned long events1;//the number of pulses from Right Motor in 50 ms;
double n1=540;//the number of pulses in one turn for the Right Encoder;
double n2=540;//the number of pulses in one turn for the Left Encoder;
//end of encoders variable

  //PID controller Variable.
 double setpoint=100; // Reference speed;
 const int Kp=10;
 const int Ki=0;
 const int Kd=0;
 double measuredRightRpm_value;//speed in RPM from Right encoder/Motor;
 double measuredLeftRpm_value;//speed in RPM from Left encoder/Motor;
 volatile signed long erorR;//Eror from Right Encoder;
 volatile signed long erorL;//Eror from Left Encoder;
 volatile signed long Eror; 
 //End of PID controller Variable.

 //Timer 2 over flow interrupt
  int x = 0, flag = 0;

  ISR (TIMER2_OVF_vect)
  {
  TCNT2=0x64;//for 10 ms
  x++;
  if(x==5) //to get 50 ms
  { 
  flag = 1;
  x=0;
  }  
   }  // end of ISR

 void eventISR ()
 {
 if (counting)
 events0++;    
 }  // end of eventISR
 void eventISR1 ()
 {
 if (counting)
 events1++;    
 }  // end of eventISR1
void setup ()
{
//setup Timer 0 and Timer 1 to generate Pwm signals
TCCR0A=0xB3;
TCCR0B=0x02; // set Timer 0 to clk/8 HZ ,fast PWM mode,tow signal inverted to each  other.

TCCR1A=0xB1;// set Timer 1 to clk/8 HZ ,fast PWM mode,tow signal inverted to each other.
TCCR1B=0x0A;;

TCCR2A=0x00;//timer 2 overflow interrupt to calculate RPM from Encoder
TCCR2B=0x07;
TCNT2=0x64;

 pinMode(5,OUTPUT);
 pinMode(6,OUTPUT); 
 pinMode(9,OUTPUT);
 pinMode(10,OUTPUT);
 //end of Timers  setup for generating PWM and interrupts.


 //Encoder Setup    
 Serial.begin (9600);
 Serial.println ();
 pinMode (2, INPUT_PULLUP);
 attachInterrupt (0, eventISR, RISING);
 attachInterrupt (1, eventISR1, RISING);
 // end of Encoder setup
 counting = true;
 // Timer/Counter 2 Interrupt(s) initialization
 TIMSK2=0x01;
 interrupts ();
 }  // end of setup

 void showResults ()
 {

 Serial.print ("the speed (RPM) for the Left Motor = ");
 Serial.println ((events0/n1)*(60000/50));
 measuredLeftRpm_value=(events0/n1)*(60000/50);

 //Serial.print ("I counted for the second  Motor  ");
 //Serial.println (events1); 
 Serial.print ("the speed (RPM) for Right Motor = ");
 Serial.println ((events1/n2)*60000/50);

  }  // end of showResults

 void PID()
 {

 measuredRightRpm_value=(events0/n1)*(60000/50);//speed in Rpm from Right/encoder/Motor.
 erorR= setpoint - measuredRightRpm_value;//Erorr From_Right Encoder.

 measuredLeftRpm_value=(events1/n2)*60000/50;////speed in Rpm from Left/encoder/Motor.
 erorL= setpoint - measuredLeftRpm_value;////Erorr From_Left Encoder.


 }

void loop ()
{

counting = true; 
if(flag ==1)
{
  showResults();
  PID();
events0 = 0;
events1 = 0;
erorR=0;
erorL=0;
flag = 0;
}  


OCR0A=192+Kp*erorR; //75% DutyCycle  for Right Motor .
OCR0B=192+Kp*erorR;
OCR1A=-192-Kp*erorL;//75% DutyCycle  for Left Motor.
OCR1B=-192-Kp*erorL; 
}  // end of loop