So the company that I work for has a system that we use to check to see what days are available, and every time that I request a day that I see open, it gets denied. Last week, I requested to take a day off in July and it got denied. My coworker buddy also asked for the same day off the following day because the calendar was showing that it was open and it got approved. I now have proof that the system works against me. There's a person that oversees this system and it seems like he modified the system so that my days get denied. I confronted the person about this via our messaging system at work and he wouldn't answer my questions. What should I do about this? My manager doesn't seem to care. Not sure if I should go to upper management or HR?

I work for a big US fortune 500 company.

Located in the US, CA