Metronome / Clicktrack not playing through headphones

[idowdell]

I am hoping that someone can help me with a problem that is driving me nuts!

On my Windows PC I can run a metronome through the built-in speakers or to Bluetooth headphones without problem. The source can be a web page, a downloaded app or click track in Pro Tools.
If I plug in headphones using the USB to 3.5mm adaptor supplied with my (Asus) computer, I only get the click playing if another audio source is playing at the same time. If I play the metronome by itself then I get nothing. If I play, for example, a sportify track at the same time then the metronome kicks in. but stops as soon as the other source stops.

It has worked OK in the past (if I remember correctly) so suspect that some setting change or corruption is to blame.
I have tried re-installing the ASUS DAC driver but this did not help.
I am hoping that someone here can help.

Thanks, Ian