Make sure that you contact your referees BEFORE you request a recommendation letter. Only send them the request if they have agreed to support your application and are aware you will be sending them an invitation link.
There can be several reasons why your referee did not submit a recommendation letter:
1. Your referee did not receive your invitation email
Most likely, the email has ended up in their spam (or junk) email folder. If your referee cannot find your invitation email there, you might have misspelled the email address. Check on your application dashboard if the email address you have entered for your referee is correct.
2. Your referee did not yet have the time to write the letter
You can send a friendly, automated reminder to your referee by clicking the "Send reminder" link on your application dashboard next to the referee's name. Please note that the number of reminders you can send is limited.