By most model codes you must have a minimum pressure at each fixture to operate properly.
If your pressure is this depilated You can call your local plumbing department/board of health and complain.
There are several precautions we Master plumbers must follow when installing any booster pump so as not to tax the water supply system causing a possible negative pressure like this sounds like.
Have a licensed plumber Or the building management document this pressure drop THEN you have the ammo to scream like heck
