Probablemente el id company esté en uso para otro elemento y esté trayendo conflicto. Probá reemplazarlo por otro. Te digo esto porque me descargué un ejemplo de la web de selectbox y probé de instanciarlo como te dije y funcionó perfectamente.
Te lo adjunto por si te es de utilidad:
Código HTML:
<!doctype html>
<html>
<head>
<title>jQuery SelectBox plugin</title>
<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
<link href="css/jquery.selectbox.css" type="text/css" rel="stylesheet" />
</head>
<body>
<select name="country_id" id="country_id" tabindex="1">
<option value="">-- Select country --</option>
<optgroup label="North America">
<option value="1">USA</option>
<option value="9">Canada</option>
</optgroup>
<optgroup label="Europe">
<option value="2">France</option>
<option value="3">Spain</option>
<option value="6">Bulgaria</option>
<option value="7" disabled="disabled">Greece</option>
<option value="8">Italy</option>
</optgroup>
<optgroup label="Asia" disabled="disabled">
<option value="5">Japan</option>
<option value="11">China</option>
</optgroup>
<option value="4">Brazil</option>
<option value="10">South Africa</option>
</select>
<select name="country_id2" id="country_id2" tabindex="1">
<option value="">-- Select country --</option>
<optgroup label="North America">
<option value="1">USA</option>
<option value="9">Canada</option>
</optgroup>
<optgroup label="Europe">
<option value="2">France</option>
<option value="3">Spain</option>
<option value="6">Bulgaria</option>
<option value="7" disabled="disabled">Greece</option>
<option value="8">Italy</option>
</optgroup>
<optgroup label="Asia" disabled="disabled">
<option value="5">Japan</option>
<option value="11">China</option>
</optgroup>
<option value="4">Brazil</option>
<option value="10">South Africa</option>
</select>
<script type="text/javascript" src="js/jquery-1.7.2.min.js"></script>
<script type="text/javascript" src="js/jquery.selectbox-0.2.js"></script>
<script type="text/javascript">
$(function () {
$("#country_id").selectbox();
$("#country_id2").selectbox();
});
</script>
</body>
</html>