3way switch not working properly
It doesn't matter if new or old but it sounds like one switch is wired correctly the other has a traveler and common reversed (quite common DIY mistake). The one plus here is by swapping one of the travelers and common it won't cause any hazards and may fix your problem. The common on the switch is usually a black screw the travelers are brass. Do this with the circuit breaker off and give it a try, if it only changed the switch position try the other side as the first try just swapped travelers the second try should do the trick.