Sometimes you can hear a "ding" but not the "dong" as the chime is weak and does not have much power. 


If using battery power then check the battery is good and new, also check it is Alkaline or Lithium type. 

Otherwise check the cable length from chime to bell push and back is under 20 meters. 


If using transformer power then check the transformer output is a minimum of 8 volts 1 amp. You can use a maximum of 12 volts. 

Otherwise check the cable length from chime to bell push and back is under 20 meters.


If using mains power (such as model 776) check the cable length from chime to bell push and back is under 20 meters.