I'm not aware of any issues with popups when using two monitors. Do the popups work if you only have a single monitor connected?
If so, please tell us what operating system you are using, and what DPI settings you use on each screen (e.g. in Windows you can use the 'Display' settings page to set the magnification to higher than 100% so that things are easier to see on high resolution monitors).
If the popups don't work on a single screen either then you may need to link them to a group or stat so that the Hud knows when to trigger them. Look for 'Assign Popups' in
this guide.