No water following through the unit when switched on at the shower?

you could have a air lock in the pipe comming up from the mains, you dont say if its tank feed or mains feed, as the mains can go to a tank in the attic as well and then back down to the shower, you could try hanging the shower hose down into the shower try, turn the know on to shower to on, but dont have any power going to it, keep it like that for a wile, see will any water drip out of the hose, if it starts to drip let it foe a wile, then try it with the power on, i would say its a airlock,

n
March 2015