I need to turn on another tap to make my shower work. Why?

[MrWalshy]

More often than not, when I switch on the shower, it will not start. I get a few drips. However if I turn on another tap or flush the toilet, the shower will start.
I'm not sure how to describe this type of shower but the photo of the main unit is below. It has a hot and cold water supply coming into it form the bottom and then the two pipes out of the side feed the overheard and handheld shower.
It is controlled by a digital panel in the shower. No other issues with cold or hot water in the house, including a regular mixer shower in the other bathroom.

 

[denso13]

Not enough flow at the shower, if you take the shower head off does it work?

 

[accelerator]

Have you asked the manufacturer? Vado by the looks of it.