2004 tire sensor monitor coming on

i followed the instructions regarding resetting the monitor for the tire sensors because i installed 20 tires, but the tire height is the same, when reseting the the monitor, i can hear the horn beep on the front left front right and the rear right, but when it comes to the left rear,,,the last one, it wont set the horn off...could this be a bad sensor,,if so, is there a way to test it..thanks..