SSL and Form Issue on my website

Hello.

I was wondering if anyone could help me.

First off, let me say that I am completely new to SSL and have a lot to learn, so please bear with me if I am incorrect or not explaining correctly.

I am working on a website that has a registration and update form. When certain users visit the site using HTTPS, the page loads but they cannot see the form. When they use HTTP, the form is visible.

Could anyone please give me any indication or reason what could be causing this? I have tried a Google search but could not find any results that gave me an answer.

I hope this makes sense and thank you for any help!