If you live in an old building at the higher floors, the water pressure may not be enough. We installed water pressure pumps for about 500rmb per pump. Do check to make sure it's not the shower head or a clogged pipe that's causing the problem.
I personally had issues with B&Q in trying to get the right parts for our bath tub (you can search the forum for the short and long of it). The customer service was atrocious and we had to threaten legal action to get anything done.
When we installed our shower, the guy from Sunning didn't dare mount it on our wall, so we had to get a handyman (Xiao Hui) to install that.