The two main things to check on the chime unit is the chime coil plunger return foam cushion. Once the foam gets old it will turn sticky and hold the plunger down and not let the coil pull it up and hit the chime bar. Replace the cushion with weather stripping about the same thickness. The other problem is the coil sleeves and plungers need to be cleaned for smooth operation. The easy way to do this is unscrew and remove chime bars, unscrew chime unit , turn chime unit upside-down to remove plungers, clean plungers and sleeves and re assemble.