page is not responsive

On this page, I have used bootstrap to make it responsive.

Eg. In the header

<div class="cta-holder col-12 col-sm-5"> (Microsoft Azure training section)

and in the testimonials also

<div class="client-testimonials col-12 col-md-6">

But this page is not responsive at small widths.

Somehow col-md-* overrides the col-*

Please suggest