CSS won't work on mobile but on pc

I'm working on my website and just finished my contact form for mobile. Everything works perfect on my Desktop when I adjust the size of my browser but not on my cellphone. Here is some code and Images :

Desktop : That's how it should look on the cellphone Mobile : That's how it looks on mobile If you need some code to answer just tell me what you need!

Thank you really much for your help, have a great day!