I replaced one leg of 3-way switches pair because I thought it was broken (only 1 switch turns on the light; the other one doesn't ==> this other one is the one I replaced). And now, both 3-way switches must be in ON position in order to turn on the light.

I tried swapping the black wires on the NEW switch, and this made me unable to turn on the light at all. So I swapped the black wires back to the original position (meaning: this position is correct, ie. I'm sure which one is common and which one is the traveler).

So does it mean the OTHER (unchanged) switch is bad? Or the wiring on that switch is wrong? (If it's wrong, how come it worked before?)

I replaced one leg of 3-way switches pair because I thought it was broken (only 1 switch turns on the light; the other one doesn't ==> this other one is the one I replaced). And now, both 3-way switches must be in ON position in order to turn on the light.

I tried swapping the black wires on the NEW switch, and this made me unable to turn on the light at all. So I swapped the black wires back to the original position (meaning: this position is correct, ie. I'm sure which one is common and which one is the traveler).

So does it mean the OTHER (unchanged) switch is bad? Or the wiring on that switch is wrong? (If it's wrong, how come it worked before?)

--EDIT/CONCLUSION--

So after unplugging all wires from switches to test continuity, I re-plug those wires.

Before: the wires were using push-in to the switches Now: I use j-hook to the terminal screws

And believe it or not: it now works!!!

I'm still trying to understand why... seems like the push-in connection is unreliable and somehow can "break" when flipping the breaker...?...

Anyway. I'll close this question and give the point the only answer so far.

Thanks, and sorry for taking everyone's precious time (blame it to the push-in)