my aqualisa digital shower turns itself off how can i stop this ?

When turning on my shower it shuts down almost immediately. My control box/pump is in the loft and can be reset by switching off and on again. I have done this twice now but need a permanent solution to this problem. The unit is approx. 18 months old as I had to replace my original box which lasted about 6 years. Could it be too cold in the loft??? Hope someone out there can assist. Many thanks