Aqualisa Quartz digital - low shower pressure?

My plumber has fitted a new Quartz shower to my mains pressure system. The old shower - Aqualisa worked very well. Another Aqulisa in a second bathroom shower works perfectly, but the new one has poor pressure. The system is fully pressurised and has a Worcester system boiler, an Ariston mega flo tank and the water pressure is very good, all supplied by a new 25mm water main. The plumber can't understand why a standard Quartz digital is not supplying the pressure experienced on all other outlets and just scratches his head! I am frustrated by the bullsh*t I am getting. Has he fitted the unit correctly?