Thats a strange one, have you kerbed your wheel and maybe there is a gap or something? If not then there is obviously some leak within the tyre somewhere, or its fitting badly to the wheel for some reason. Give them some stick until they fix it!
That may be the case. These are brand new tires I had put on by the dealer 2 weeks ago. The problem is the dealer won't do anything about it. They say they can't find the leak so everything must be fine. Now they're just sitting on the car taking pressure readings every day. I've told them that they should drive the vehicle but they won't.I had the same problem with pirelli runflats on my 04 MCS, it's because the sealant around the inner edge has broken (probably cornering a little too fast - the hard side walls of the runflat, break the seal! They sometimes only let air out when cornering. They simply need re-fitting!