Making a contact form mobile friendly

I have a website where I incorporated a ready-made contact form PHP into it with a bit of modification. But when the contact form page is displayed on a mobile phone or when browser gets narrower, the form input fields and some texts don't get flexibly adjusted to the browser width, and a part of input fields and some texts are cut out from the browser. The page doesn't even allow you to scroll horizontally. I tried to adjust it by modifying some codes in "@media only screen and ( max-width: 480px )" section, but it didn't work. I tried to make the container field of texts flexibly by setting the width 100% when the browser width is shrunk, but it didn't work. So I have no clue what's causing to fix the width. Of course, it has to remain at a certain width when monitored with a browser with wider width, but I need to make it flexible when shrunk so that it works with mobile phones. I can either make the full website pages downloadable for you to edit, or issue a temporary ftp access to you. But I'm strictly against misuse of the contents and break of security. As far as I see, the project may look very easy, but not as easy as just a simple modification of CSS. Because the form PHP is not mine, and I think it's something to do with the program. The contact pages comprise three pages, input page, review page before submission, and after-submission page. So all three pages have to be modified. Thank you!