When using the mechanical chime, you need to use the Mechanical Chime Dongle ADC-VDBA-MCD (included). Has this been connected?
Installation instructions can be found below to review:
The recommended transformer rating for an AC transformer is 16-24 VAC, 10-40VA (16VAC, 10VA recommended). You will want to check to confirm that is what you are using.
Alternatively, a DC transformer 18-24VDC, 12-30W could be used to power the ADC-VDB75. While we do not carry it at this time, the Alarm.com ADC-VDBA-PSU-DC could be used as a plug in option.