I just slept only to find out that I skipped a single line on a head that made my client really unhappy. Task was to make his website responsive, after all the design, forgot to add following in its head. I hope that it would save you a lot of embarrassment. You are most probably missing a viewport meta field too. You can paste following to your <head> and it should start working.

<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">