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 ->

You may be using an intermediate redirection technique such as this:

Google Ad -> Click -> -> Automatic redirect to

In the above scenario, you may not have the Call Dynamics JavaScript code running on the site '', but you will have it running on '', and so all Call Dynamics can see is the traffic being referred from to

To fix this, you'll need to ensure that the JavaScript tracking code is running on your subdomain, and in your Call Dynamics tracking Pool configuration, click on the 'Website' tab and enable 'Allow tracking on multiple domains', then click Save.

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.