Please look at this example. It works fine.
<!DOCTYPE html>
<html>
<head>
	<meta charset="UTF-8">
	<title>Load Form Data - jQuery EasyUI Demo</title>
	<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/default/easyui.css">
	<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/icon.css">
	<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.min.js"></script>
	<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.easyui.min.js"></script>
	<script>
		$(function () {
			$('#fm').form('clear');
			$('#fm').form('load', {
				name: 'name1',
				weekdaysList: ['2', '3']
			})
		})
	</script>
</head>
<body>
	<div id="dlg" class="easyui-dialog">
		<form id="fm" class="easyui-form">
			<input class="easyui-textbox" name="name" label="Name:" labelPosition="top">
			<div id="weekdaysList" name="weekdaysList" class="easyui-checkgroup" data-options="
				labelWidth: 100,
				data:[
					{value:'1',label:'Monday'},
					{value:'2',label:'Tuesday'},
					{value:'3',label:'Wednesday'},
					{value:'4',label:'Thursday'},
					{value:'5',label:'Friday'}              
				]"></div>
		</form>
	</div>
</body>
</html>