One possibility with old speakers is that the capacitors in the crossover may be failing. I once got hold of a pair of speakers with no sound from the tweeters. I tested the tweeters individually, and they were fine. The capacitors in the crossovers had failed, causing an open circuit, so none of the signal was getting to them. I replaced the capacitors, and then all was well. First, though, try the speakers with another receiver. If they don't work right with another receiver, you might want to test the tweeters by themselves, though you will want to be careful, because they are not designed to get bass and high levels of bass will likely destroy them.
I just had another thought: Do your speakers have any level controls on them? If they do, make sure the tweeter control is not turned all the way down. You might also want to move the controls back and forth a bit, as they can get dirty and cause the signal to be diminished or eliminated. If that is the case, clean the controls with DeoxIT.
And, of course, you should also make sure a high filter on your amplifier has not been switched on, or that the treble has been turned all the way down.