Is it possible that your form and landing page are set up to use an Incentive Email, and the folks haven't confirmed? If that's the case, when you go to your subscriber list and change the list from "Confirmed Subscribers" to "Unconfirmed Subscribers" you'll see them listed there.
If that's not what's going on, you should reach out to ConvertKit's support team at help@convertkit.com58 (or with the message bubble in the lower right corner) as there may be something going on with your account.