I guess it's low water pressure. If someone is taking a shower you can't even wash your hands in the sink because there's barely any water coming out. It could be the bathroom sink or the kitchen sink. If someone flushes the toilet you loose almost all the water in the shower for a few minutes. I've checked the water valve at the street. It's turned all the way up. What else should I do? It's becoming annoying. Does a water softener tank affect water pressure?