select2 is not showing arrow down icon

For Select2 version 4.0.5+, this code exactly duplicates the single-select arrow.

.select2-selection--multiple:before {
    content: "";
    position: absolute;
    right: 7px;
    top: 42%;
    border-top: 5px solid #888;
    border-left: 4px solid transparent;
    border-right: 4px solid transparent;
}

You have used multi-select dropdown and used plugin(Select2) don't add dropdown arrow in multi-select dropdown. So if you want to add dropdown arrow, you have to add some custom CSS like below.

ul.select2-choices {
    padding-right: 30px !important;
}

ul.select2-choices:after {
    content: "";
    position: absolute;
    right: 10px;
    top: 50%;
    transform: translateY(-50%);
    border-top: 5px solid #333;
    border-left: 5px solid transparent;
    border-right: 5px solid transparent;
}