چرا لایه بندیه بوت استرپ کار نمی کند

[charcharkh]

• .col- (extra small devices - screen width less than 576px)
  
• .col-sm- (small devices - screen width equal to or greater than 576px)
  
• .col-md- (medium devices - screen width equal to or greater than 768px)
  
• .col-lg- (large devices - screen width equal to or greater than 992px)
  
• 
  
• .col-xl- (xlarge devices - screen width equal to or greater than 1200px)

منبع :

https://www.w3schools.com/bootstrap4...grid_basic.asp