Generally, that means you’re running into a conflict with another plugin or your theme

Take a look at your browser console and see if any javascript errors are being thrown in there. 

Also, you can try switching your theme temporarily to see if the issue goes away.  If not, disable other plugins as well.  The combination of the two steps will confirm if you have a conflict.  Don’t forget to flush your WP caches (if you have one) and your browser cache after disabling or enabling an item.

Thanks.