Funraise donation form is partially hidden

I have had a few donors reach out that they are unable to complete credit card donations because they can't access the entire donation form each time they click through. I cannot recreate this issue, it works fine for me. It doesn't work for everyone. Funraise and Squarespace both say it's not their issue. One of the customers sent me the screenshot attached. Funraise said: Your web developer will want to adjust the settings so that the entire form is visible from each step in the donation process. 

Any help is much appreciated! We are a small nonprofit and don't have a web developer, and we haven't had this issue in the past. Not sure what changed. 

Could you share your website URL?

How is this donation set up?

Yes, sorry. It's miraclemessages.org/donate. It's a Funraise form.

The iframe height is set to 600px, try changing that to 700px.