2003 Ford Explorer AC Problem

Hi
I have a 03 ford explorer with 60,000 on it. I'm having a problem with ac coming out cold on the right vents and warm on the left. The temperature seems to rise from right to left. I just moved to phoenix, az and refilled my ac with the refrigerant 134a and it seemed to last about 3 months. I'm not sure how long the refills are suppose to last but it seemed to go out pretty quick. Today I refilled the refrigerant with a refill that has leak sealer but it doesn't seem to fix the problem with the left vents being warmer. Any ideas on what my problem might be?