Image grouping not working

Hello,

I have problem with grouping images into some groups.
Simply it is not working the way I do it.

Here is some code i use.

<div id="container" class="isotope gallery">
<a class="grid-item" data-filter="city" href="someimage.png" rel="rel1">
<img src="someimage.png" alt="" title="" /></a>
<a class="grid-item" data-filter="cats" href="someimage.png" rel="rel1">
<img src="someimage.png" alt="" title="Beautiful Image" /></a>
</div>

So as you can see there is attribute rel in a tag.

In plugin setting I have changed grouping to true from default false.

What am I doing wrong? Could you please help me?

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