I take it that the rim which was previously on the driver's side is now not losing any pressure on the passenger side?
I am at a loss really as for what to suggest. When exactly are you checking the tyre pressure? Immediately after driving the car, a good few hours after, the morning after etc.? The pressures will vary a little as the car is driven and the air heats up depending on the roads you drive on etc. but this should settle down once the car has cooled down. Are you checking the pressure at a petrol station?