I have an event called "Global Shortages" which should trigger for all countries about 30 days after the start of the game adding the corresponding national spirit. However its just not triggering.

I'm going to assume you mean the event "Global Supply Shortages Spread", if it's not that one let me know.

If it is this one, the trigger means it isn't valid for any nation:

tag = usa

has_idea = supply_shortages

Assuming all nations have the idea "Supply Shortages" that's fine. But the tag for the United States is "USA" (capitals matters), and then the event would just fire for the US.

If you're trying to check if the US has that idea, you need:

USA = {

has_idea = supply_shortages

}