Hi there @Stephanie D.,

Thanks for sharing with the Community.

Sorry to hear that subscribers are not being added to the intended subscriber List within your Klaviyo account. I will point out that your List could be set to Double Opt-in. This means that there is a confirmation email sent to your subscribers after they submit your form but before they are added to the List. This email is also known as the Double Opt-in email.  
 
The Double Opt-in process requires action on the subscriber’s part. They will need to check their email after submitting your Signup Form, to confirm their subscription. This behavior will determine when they are added to the List, as well as when they enter any associated List-triggered Flow(s), such as a Welcome Series flow.

You can learn more about the Double Opt-In process here. Alternatively, you can change your List to Single Opt-In by navigating to your List, click on the Settings tab at the top, and then update the Opt-in Settings. Please also be sure to click Update List Settings after making any changes.

After resolving this and checking for double opt in, you could create a segment of profile, who did consent via this form and was not added to the list, export this segment and import it to your list. 

To create a segment please navigate to Lists and Segments->Create new list/segment->Segment and use the following definition: 
 

 
Once the segment is ready, you can export it and import in the list, if you need to do so: How to Export a List or Segment to a CSV File, How to Add Subscribers to an Existing List. 

Hope that helped!
Alex