Not able to control DC motor speed with l298N & pwm pins
Try to use this code, without the library, and be shure to connect every pin to the correct arduino output! If everything is ok it should work! Check a tutorial here 

int IN3 = 5;    
int IN4 = 4;     
int ENB = 3;    
void setup()
{
 pinMode (ENB, OUTPUT); 
 pinMode (IN3, OUTPUT);
 pinMode (IN4, OUTPUT);
}
void loop()
{

  digitalWrite (IN3, HIGH);
  digitalWrite (IN4, LOW);   
  analogWrite(ENB,55);
  delay(2000);
  analogWrite(ENB,105);
  delay(2000);
  analogWrite(ENB,255);
  delay(2000);
  analogWrite(ENB,0);
  delay(5000);
}