Apparently Global Variables cannot be used with scheduled runs, but I was able to work around this limitation by changing the variables to Environment Variables. Environment Variables offer similar functionality to Globals, but you just need to select the environment before you send the requests. So when manually sending requests, you need to first select the Environment at the top-right corner of the Postman page. With Scheduled runs/Monitors, there is an Environment setting there as well; just select the Environment that has the variables that are needed for that run.
I hope this helps!