Hello Jagadeesh,

Please share the Design Time Configuration of the portal header. 

Try inspect the header element using Debugger tools and check CSS styles whether any custom css style is overriding the header position.

Attach the screen shot of inspect header element  with Developer Tools enabled. 

You can try with inline css style for header as below:

 position: fixed;

or if you know the ID of the header then use below snippet in your custom css stylesheet.

<style>
#ID{
    position: fixed;
   }
</style>

I hope this helps you .

Thanks & Regards,

Durga