Q: AC works partially, depending on low or high setup

Hi. I've been having problems with the AC not working correctly. It works only partially and I need some advice on how to fix it. It changes the way it works depending on the settings. If it is set up on high, the vents on the passenger's side blow warm air, but the driver's side gets cooling. As for the low set up, the vent on the back seat and the vent of the front windshield blow cold, whereas the front vent doesn't work at all when set on low. Do you have any suggestion how I could fix this? Thanks.