Newbie Advice sought

Hi Everyone. I am running an ad campaign for my construction business.

I have a basic google ad campaign going which leads people to my Wordpress Site.

There is an opt in form there. Those who opt in are added to a contact list in Brevo and are sent a welcome email inviting them to reply with a bit more information.

A very small percentage open my welcome email. Are these welcome emails going to spam? Were the responders not real in the first place? What might be happening here?

I am obvious new to email marketing and recognize that I have a lot to learn. I would be so appreciative of any advice offered.

Thank You and All The Best!