tire pressure

Hello All, Just purchased a 2008 LT, 67k and have some issues. The Service Tire Monitor System message is on at startup, and then going through, NONE of the sensors seem to be reading, all i see is -- Took it back to the selling dealer they said " we couldn't reset it"..typical To add...

The tire sensors on my 07 Tahoe worked fine then I rotated the tires and magically the service tire monitor light came on. I have tried resetting them per the manual 10 times why do they magically go from working then when you try to relearn them they don't work. I enter the learning mode and...