Footer widgets missing on phones

i noticed the footer widget area is not displayed on smartphones (portrait & landscape) and tablets in portrait mode.
Does this require any setting ?

Both the sidebar and the footer widgets are hidden by default on small devices. If you want to turn them on use this please:

@media only screen and (max-width: 991px)
  .widget-area, .footer-widget-area {
    display: block;

I also face the same problem and still invisible even modify the css with Vlad mentioned above.
Is there any setting require me to do?