I recently tried replacing a pair of three-way switches in my home. They were very old (ceramic housing!) and didn't have the indicative single dark screw and two brass screw terminals.
Now, it seems that only one switch at a time can operate their common light fixture. To clairify, here's a scenario:
If the light can be turned on with the upstairs switch, that same switch can turn it off. However, if the light is turned on with the upstairs switch and turned off with the downstairs switch, then only the downstairs switch can turn the light back on. Basically, only the switch that turns the light off can turn it back on again.
Can anyone give me a clue as to what's going on?