why my ultra2 watch does not notifying when left behind

I have an iPhone 17air and already have my ultra 2 watch, this iPhone is new I pair every device on it except the watch it kept on can't mention to notify me when left behind .. I do all the steps required on watching and asking YouTube or apple support or google and still remaining the same. I reset, unpair, and pair again I do it several times still same. what shall I do ?