I send my cousin a friend request, but he says that he hasn't received it. I send it again a few weeks ago again and he says the same thing. Can you tell me what's happening?
After you have clicked on the "Add Friend" button on someone's profile to send a friend request to them, that button will change to "Friend Request Sent":
That person then has the option to either Confirm your friend request or Ignore it:
If they click on the Ignore button, then when you go back to their profile, you will see the "Add Friend" button again instead of "Friend Request Sent".
So I believe your cousin does not want to be Facebook friends with you, and is clicking on Ignore every time you send him a friend request. You can verify this by sending him another friend request and then going back to his profile and seeing that the "Add Friend" button changes to "Friend Request Sent". Then keep checking his profile once in a while to see it change back to "Add Friend" when he ignores your request.