Depends on the shape of your evaporator and the way the air flows, but when one side of the car is colder than the other, it could mean that the freon is low again. When you are low on freon, a weak stream of freon comes spitting in to one end of the evaporator, and only that end of the evaporator gets cold, so only that side of the car gets cold air. If this theory is right, soon you will have so little freon that the system won't have enough pressure for the compressor to engage, then you will have no cold air at all.
Freon leaks are not hard to find. You need a new shop.