Likely just low on refrigerant. Leaky. I have to top mine off every now and then. It's always colder on the pass side because when it's low on refrigerant it doesn't have enough to cool the whole coil, so it's warm before it flashes off on the driver side of the coil. Or something like that. When it's 90+° out it takes 10-15 minutes of driving before my driver vents are cold. Just not motivated enough to top it off right now.