If you are building your Bootstrap 3 grid entirely using mixins, you need to declare a base column mixin for proper, full width columns at the smallest level. Otherwise, they lack the proper padding, min-height, and position as the same HTML markup column would have by default. This is used by adding .make-column(); at the base level before you …

Bootstrap-3-Smallest-Column-Mixin

// Generate a mixin for mobile full width columns, to maintain the same attributes
.make-column(@gutter: @grid-gutter-width) {
  position: relative;
  // Prevent columns from collapsing when empty
  min-height: 1px;
  // Inner gutter via padding
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);
}

// How you use this

.this-is-your-row {
  .make-row();
  .this-is-your-column {
    .make-column(); // This makes the smallest column have the right padding
    @media(min-width: @screen-tablet) {
      .make-md-column(6); // This is what you wanted for your grid
    }
  }
}