Displaying Google Map in a Bootstrap Modal

I have no problem displaying maps in posts or pages but the map fails to display when included in a bootstrap modal pop-up.

The copyright and zoom controls display but the map area is just a grey background.

I am using coding found here:
http://www.w3schools.com/bootstrap/bootstrap_modal.asp

Any help much appreciated.

Thanks
Tony

https://ww.wp.xz.cn/plugins/responsive-google-maps/