Xstream Hot Water Cylinders - Paarl

Our company Solar Dot Com has purchased 2 150L Exstreme geysers at Sunscan last year November and installed them with 2 solar flat plates. The installation were both a pump system and they were in series. Both systems were installed on top of a double storey. The problem I am facing after installation is that when running the hot water through out the house there is a drop in pressure on the hot supply to the taps. The plumber and our company has run a few checks to see whether we have done something wrong. We have disconnected one of the systems and tested it but the problem still persist. We have changed 3 master valves (400kpa) and tested it but the problem still persist. We finally disconnected both systems and feed a cold water supply at 400kpa to the the hot water line and tested it and to our surprice both pressure was exactly the same. Please asssit me in this regard as matter of urgency as I am facing a furious client.