Ford Focus loss of tyre pressure, no leaks! on new focus

If the tyres are not leaking it must be a fault with the TPMS ?

I have had everything checked etc by a good tyre shop. It is in service with Fords, they cannot find a problem. I have seen on other web sites that the wheels can be inferior / defective.
The car is still under warranty (two people on the other sites had the wheels changed by Ford, under warranty!) I am collecting the car tomorrow, not sure how I go about it, but getting prepared to complain / claim under warranty!
Thank you for your reply, Mike

I know of some where corrosion to the alloy wheel where the valve passes through causes a very slight leak . I seen this also with a wheel that was in a tyre bath for at least a minute before up popped an air bubble . Sometimes tyre fitters dont take enough time , into the bath and back out with the wheel - no leak .I believe there is a valve sealer kit used after old valve with sensor replaced after cleaning up where the valve fits in wheel and sealer also applied .If you can get new wheels under warranty , all the better