CDK virtual scroll has no doms

I'm using CDK virtual scroll to improve the performance of a dropdown which will have huge data. I have followed all the steps as per the documentation. But when I open the dropdown I don't see any dom with in cdk-virtual-scroll-viewport component.

Here is the code from HTML.

<cdk-virtual-scroll-viewport itemSize="500" class="example-viewport">
            <div *cdkVirtualFor="let item of items" class="example-item">{{item.name}}</div>
        </cdk-virtual-scroll-viewport>

If I move the div outside the cdk virtual scroll and replace it with ngFor it works fine. Any idea what could be the issue?

The problem was that the scrolling module was not imported in the correct module file. It didn't give any error but was giving just a warning.