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Pressure variations in different parts of a home can almost always be found within the fixtures themselves. Sometimes, a generalized pressure problem can be traced to nothing more complicated than a well-overdue whole-house water filter replacement! However, the variations throughout your home probably can't be attributed to a single, localized problem.
Newer fixtures have various flow restriction devices mandated by the Feds to reduce water use. If you purchase a new showerhead, for example, and are used to an old one, you will be amazed by the loss of water volume these newer fixtures produce. Some have removable flow restrictors, some incorporate permanent restriction. Permanent flow restrictors can sometimes be enlarged by deft drilling of the restrictor. This should be done with much caution to prevent damage to the fixture.
We can't rule out what I fondly call "Amazing Plumbing!"… amateurish pipe installation that features varying pipe sizes to use up whatever the plumber or homeowner had laying around in the basement, over -soldered joints which affect water flow, etc. So a thorough examination of your system may reveal the mystery bottleneck! And don't forget to check the shutoffs… the "last guy" may have intentionally or unintentionally closed a shutoff and not fully reopened it!
With sink faucets and showerheard, the two most common causes of low pressure are 1) one or both of the shutoffs are partially closed or 2) the aerator screen/filter screen has become blocked with dirt particles. (This graphic shows a showerhead filter screen blocked with well sediment) Sometimes the aerator screen can be cleaned, though replacing it is often a better choice. It the aerator is old and has not been disturbed for years, chances are you will damage it during removal. In either case, replacement of the aerator with a new one is an inexpensive and complete repair.
Do not under any circumstances remove the screen and not replace it! Once this grit gets into a showerhead it is difficult or impossible to clean out.
It is unlikely that your neighbors' water use would affect your water flow. The size of the pipes used for municipal water supplies is large enough to keep the flow even to all homes, even if everyone turned their taps on at once. I would contact the company that manages your public water supply to see what experience they have with this problem.
Generally speaking, a draw on your water system in one part of your home will affect the pressure elsewhere, depending of course on the size of your home's pipes, the starting pressure and the volume of water flow. A garden hose, for example, will have a more dramatic effect than a toilet, since the volume of moving water is greater. Also, there are "automatic" systems that might cause temporary draws you might not be aware of at the time the pressure drops, such as self-cleaning water softening systems, sprinkler systems or automatic refilling of a heating system.
If you happen to be on a well system, variations of water pressure are normal. Some folks who are accustomed to the constant pressure of "city water" are astounded at the difference! Showers have to be carefully planned around toilet flushes, dishwashing and, of course, the laundry. Or else there may be lots of screaming!
Residential well systems operate within a range of pressures. Keeping the pressure absolutely constant would require the pump to cycle on every time you drew water, increasing the wear on the pump dramatically. This would also cause you to experience a "pulsing" in the water as each cycle of the pump moves water.
Popular Mechanics has good information on how a well pump works at this URL:
You should also investigate whether your home has an automatic pressure adjusting system. Some municipal water supplies have such high pressure that residents install pressure-reducing equipment. A malfunction with this system could possibly cause unwanted pressure variations.
Of course, there could be other hidden factors I have missed. In which case you might want to have a plumber inspect your system for other pressure "bottlenecks".