Bootstrap4 dropdown only works in the second click

you need to use the toggle like that and assign the data-target to the drop down menu with it's id

using

data-target="#navbarDropdown"

 <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarDropdown" aria-controls="navbarDropdown" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
      </button>

and then assign that id to the menu

        <li class="nav-item dropdown show">
            <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarDropdown" aria-controls="navbarDropdown" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
      </button>
            <div id="navbarDropdown" class="dropdown-menu"  role="menu" aria-labelledby="navbarDropdown">
             <a class="dropdown-item">Link 1</a>
             <a class="dropdown-item">Link 2</a>
            </div>
        </li>

I came across the same issue recently, i solved this issue by adding $('.dropdown-toggle').dropdown() after loading the drop-down. Hope this may help without changing from the data-toggle="dropdown" to data-toggle="collapse".

I had the same problem then resolved. Often I due to a double import of bootstrap js libraries. Hope will help you.