I know it may be a stupid question but you do have the USA maps installed ??
Only asking as when we were putting mine in the car last November a guy came up asking for help as he could not get his working, turned out he only had maps of UK / Europe and did not know he had to buy US maps seperate.
Plus be prepared for it to take a long time to lock on to a signal the first time you turn it on after arriving, dont think it is broke, it is just looking in the wrong place and takes far longer to scan, will be fine after that.
Graham