Adjusting well pump pressure
The problem you are describing is one of a lack of pump capacity. If when the water is being used in the house, the pressure cannot get past 40 PSI, it's because your pump is maxed out. If it USED TO work and has become worse, you may have a pump problem that should be looked at, i.e. sand infiltration, low water table, worn out impeller etc. etc.. If it has ALWAYS been like this, then your pump is too small, or rather too small to give you any more.