Power Shower

My power shower which was installed a week ago is a Salamander 1.5 pump which is under the bathtub.
When we switched it on the power of water coming through the shower is totally inadequate, even worse than an ordinary shower.While having a shower the water force reduces even more.
Should I change the pump to a 3.5 or is it anything to do with the water pressure in my house.
Please help

What is the water pressure in your house? How old is your home? What material are the pipes made of? Did the Power Shower ever work when first installed? I've read the specs on your pump. Their warranty reads, " 2 Year Manufacturer's Guarantee Extendable to 3 Years." They claim your power shower will supply two showers with 10 ltrs. Per minute Have you contacted them yet? Have you removed the shower head and tried the pressure then? Back to you. Tom