arduino: program with else not working

#include <Servo.h> 

Servo myservo;  // create servo object to control a servo 


int pos = 90;
String kontrolstr = "";
char kontrol;

void setup() 
{ 
  Serial.begin(9600);
  myservo.attach(9);// attaches the servo on pin 9 to the servo object 
} 

void loop()
{
  if(Serial.available())
    {
      kontrol=Serial.read(); // it reads from python voice recognition
      kontrolstr.concat(kontrol); 
    }
     if(kontrolstr== "right")
       {pos += 30;
       kontrol = '0';
       kontrolstr = "";
       }
     else if(kontrolstr== "left")
       {pos -= 30;
       kontrol= '0';
       kontrolstr = ""; 
     }

     myservo.write(pos);
     delay(100);
}

It works with voice_command.py (which I wrote) on linux terminal. When code is like this, right after uploading this code to arduino, it works well until voice recognition understand a different word from "right" or "left". When voice command send to arduino another string different from "right" or "left", program still works without any error but after this point it starts not to respond "right" or "left" command anymore. To solve this I did this change. I put an 'else':

#include <Servo.h> 

Servo myservo;  // create servo object to control a servo 


int pos = 90;
String kontrolstr = "";
char kontrol;

void setup() 
{ 
  Serial.begin(9600);
  myservo.attach(9);// attaches the servo on pin 9 to the servo object 
} 

void loop()
{
  if(Serial.available())
    {
      kontrol=Serial.read();
      kontrolstr.concat(kontrol);
    }
     if(kontrolstr== "right")
       {pos += 30;
       kontrol = '0';
       kontrolstr = "";
       }
     else if(kontrolstr== "left")
       {pos -= 30;
       kontrol= '0';
       kontrolstr = ""; 
     }
     else {              // I write this to make it work..
       kontrol = '0';  
       kontrolstr = "";
     }

     myservo.write(pos);
     delay(100);
}

However, now it is not responding "right" and "left" command too. How can I solve this problem?