I'd also check if some of those subscribers are coming in as unconfirmed (assuming you have double opt-in enabled on your form). It's possible some of the signups are bots, which would explain why they aren't confirming their subscription. We have a guide on unconfirmed subscribers here9, and on double opt-in here2.
If that doesn't seem to be the case, feel free to reach out to our team at email@example.com (or via the in-app messenger in your account) with a description of this issue, the URL on which the form is embedded, the name of the form (as it is displayed on the forms page3 in your account), and some example email addresses of subscribers you believe should have been added to your subscriber list, but were not. One of our Product Specialists will be able to help troubleshoot further! :)