If the phone is old the ringer capacitor could be open, shorted or dried out. Are the ringers concentric? Sometimes people dont want the bell to clang and they turn the bells out which clears them from the clapper. On incoming ring look at the clapper is it moving back and forth but not making contact with the bell. If it is moving a little try moving the bias spring, connected to the clapper arm, to another position.

Forty six years and still fascinated with Telecommunications!