They can be assembled with the spring between the mounting stud head and the sensor, and that will leave the sensor up against the wheel but it is incorrect. The reason for the spring and the 'too long' stud (Easy Boys!) is so that the sensor 'floats' inside the wheel. The sensor should not touch the wheel at all, and it should not be rigidly mounted (Boys!) if installed correctly. It sounds as though your rear sensor (BOYS!) might have been installed incorrectly in the first place.
BTW- they will also install 'upside- down'. That apparently happened at the last tech. day I attended; I swapped the battery but someone else installed the sensor in the wheel and I believe he got it facing the valve stem the wrong way. Actually, the sensor will work OK like that except that it will never be turned on as the centripetal force is going the wrong way when they are mounted that way.
Brian
It was your pics of a tutorial about servicing with out removing the tire that helped me compare to mine. I swear the rear one sat almost flush. Thanks all.
Wayne