Air tight

The left front wheel of my car seems to have a will of its own. It is the only wheel that lets the air out of the tire. I have changed tiers and "crossed" them but the air still seeps out - very slowly - and I have to keep an eye on it, because suddenly it has dropped the pressure from 30 to 23 - and that is not good.
If I was superstitious I would say that the car was doing this because it felt neglected and needed my attention.