DataTables FixedHeader Error

Hello,

first of all I want to thank you so much for creating such a great plugin! I use it on a feature-comparison page: mb.ac/cube-vergleich

Today I have customized the table by adding the following Custom CSS:

.tablepress tbody td {
	font-size: 14px;
	color: #333;
}
.tablepress thead th {
	font-size: 16px;
	color: #333;
}
.tablepress .column-1 {
	width: 30%;
	border: 1px solid grey;
}
.tablepress .column-2 {
	width: 35%;
	border: 1px solid grey;
}
.tablepress .column-3 {
	width: 35%;
	border: 1px solid grey;
}

and the result is as expected. The only problem I have is applying this extention correctly: DataTables FixedHeader

I have deployed the table by using this shortcode:
[table id=4 datatables_fixedheader=top /]

Inside the table the first row is activated as table-header.

Now when I scroll down the page I would expect the header-row to stick on top of the browser-window, but it does not stick.

I would be more than happy if anyone could help me with troubleshooting and solving this issue.

Have a wonderful day!
Sascha