There are several reasons why this might occur, however, the most common reason is that you are using an intermediate redirection technique to track traffic.
For the Call Dynamics tracker to correctly attribute visitors it needs to see the first 'hit' after the initial click that drove the traffic to your site, for example, the flow would look like this:
Google Ad -> Click -> https://YourWebsite.com
You may be using an intermediate redirection technique such as this:
Test again from another computer and a phone you haven't used to call in with before, and you should now see your traffic being attributed correctly.