why is my low tire pressure not coming up?

pressure will go up as air is added to tire(s). Not by itself..........Sensor(s) may have been damaged at tire changing time , is it simply the low pressure symbol , or are the 'TPMS' letters on? BIG difference. Also , sometimes certain chargers or power adapters will cause interference and turn the TPMS light on with no stored DTC. Can make programming the sensors a hassle too.