What Triggers Hazel to Run?

So, I have had multiple times where it seems like Hazel doesn't run. For example, I'm monitoring the download directory for .PDF files (bills). I downloaded a bill this morning at 7:06 and Hazel never ran. I then went into the UI and wanted to test the rule against the file. It matched perfectly. Once that was done, Hazel processed the rule a minute later.
All in all, the file was downloaded at 7:06am but the rule didn't process until 7:15am.
Any ideas?
All in all, the file was downloaded at 7:06am but the rule didn't process until 7:15am.
Any ideas?