Dropdown symbol doesn't appear in Chrome

Question

I'm currently struggling with my select box.

For some reason the "arrow-down" symbol doesn't appear when I use the chrome browser... So on IE and Firefox it is working fine.

How it looks in Chrome?

How it should look? (e.g. Firefox)

Any error messages?

Yeah, I don't know why, but I receive following error message on every browser:

Uncaught TypeError: $(...).dropotron is not a function
    at HTMLDocument.<anonymous> (init.js:76)
    at j (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at Function.ready (jquery.min.js:2)
    at HTMLDocument.J (jquery.min.js:2)

The affected Code

// Dropdowns.
        $('#nav > ul').dropotron({
            mode: 'fade',
            noOpenerFade: true,
            alignment: 'center',
            detach: false
        });

What Framework(s) / Labrarie(s) am I using ?

I'm just using jQuery and skel... maybe there is a complication there ?

Does anyone know what I'm doing wrong...? I can't find any solution nowhere.