I've also been having problems with this.

I couldn't hear any of the sensors clicking - so I downloaded a sound meter app for my phone (free) which can obviously hear Ultrasound! You could see the meter spike a couple of times a second on all but one sensor - which was completely dead!

I've ordered another sensor - so we will see! This strikes me as more reliable than listening for a clicking / ticking that I still cant here even though I know it's there!

Si