@Crea.campa Did you check the Integration Page to make sure they are associated with the correct List? Make sure you have the “Collect email subscribers” checked, and that you have the correct List selected in the Dropdown. 

See here:

Other Troubleshooting tips:

• See if your List has Double Optin enabled.  If so, a user needs to confirm in the email address before they are added to the List. 
• You can disable Double Optin if you want, but you will need to contact Klaviyo to enable that functionality if you don’t see that option in your List Settings.
• Are you looking at a Segment (and not a List)?  Make sure you know the difference between a Segment and a List.

Let us know if it worked or if you’re still having problems!