I use Mailchimp to set up newsletter sign-up form. (use ShopSync app to connect my shopify store）
But newsletter sign-up form didn't show up.
I have disabled Shopify's default newsletter sign-up form.
How could I fix it?
You can only change when the pop-up appears (immediately, after 5 sectons, on exit, etc.) You can change it in the pop-up form builder settings. However, as far as I know, you it's not possible to change the behaviour that the pop-up doesn't show up if it was closed or filled-in earlier.