@Enoch 

What I would suggest first start with your integration and make sure it is done correctly and everything is working fine there. But to be honest most new people that I talked to get this problem because of the double opt-in option.

Check your main list, If it has the option Double Opt-in turned on instead of Single Opt-in people will also have to click on confirmation email to be signed up to your list. And most people don’t do that. So maybe try turning off this option for few days and see the results, than you can decide what works for you if that was the problem. 

Btw FYI the double opt in option is good against spam subscriptions but also it makes it harder for actuall subscribers to get in the list. So at the end it should be your decsion if you want to be safe from spam subscribers or want to make it easier to people to subscribe :) 

Thanks, I’ll try this out.