Why is these flex items not wrapping?

Question

I have a flex item that is also a flex container .sub-con, problem is the flex item of .sub-con is refusing to wrap, even after adding : flex

Answer 1

Your flex items in the nested container are sized with percentages.

.col-one{
  width: 40%;
  height: 100px;
  border: 1px solid lightgreen;
}
.col-two{
  width: 40%;
  height: 100px;
  border: 1px solid red;
}

Because percentage lengths are based on the length of the parent they have no reason to wrap. They will always be 40% of the parent, even if the parent has a width of 1%.

If you use other units for length, such as px or em, they will wrap.

jsFiddle demo

 .container {
   display: flex;
   justify-content: center;
   align-items: center;
   flex-flow: row wrap;
   height: 100vh;
 }
 
 .sub-con {
   flex: 1;  /* for demo only */
   align-content: flex-start; /* for demo only */
   margin-right: 50px;
   margin-left: 50px;
   height: 500px;
   background-color: #fff;
   display: flex;
   justify-content: center;
   flex-flow: row wrap;
 }
 
 .col-one {
   width: 200px;
   height: 100px;
   border: 1px solid lightgreen;
 }
 
 .col-two {
   width: 200px;
   height: 100px;
   border: 1px solid red;
 }

<div class="container">
  <div class="sub-con">
    <div class="col-one"></div>
    <div class="col-two"></div>
  </div>
</div>