Flex - not working on mobile?

Question

Flexbox

On mobile, everything is exactly the same blocks are lined up in 1 column
, what could be the problem?

.flex-container {
    display: flex;
    flex-wrap: wrap;
}

flex-item {
    width: calc((100% / 2) - (20px / 2));
    margin-right: 10px;
    margin-bottom: 10px;
}
.flex-item:nth-child(2n) {
    margin-right: 0;
}