Video Doorbell Pro installation issue

I am trying to install a Video Doorbell Pro; however, when it is all connected up, my current chime buzzes continuous and nothing happens when I press the button on the doorbell.

My current chime is a continuous ringing type (a bell) so when the circuit is closed, it rings continuously until the button is released. I have checked the wiring to the button and have confirmed that this is good - when I short out across the contacts on the Video Doorbell Pro backing plate with a piece of wire, the bell rings as it should. However, as soon as I slot in the Video Doorbell Pro, the chime starts to buzz and the bell does not ring when I press the button. I have checked my transformer and it is producing a voltage of approximately 9v AC so that is within the required range so I don't think the power supply is an issue.

Any suggestions as to how to resolve this and make the doorbell work would be appreciated.

Thanks.