As pipes leak or clogs move around inside the pipes water pressure can vary and fluctuate.
Well pump water pressure varies.
Basic well pump water pressure guide if you live in a highly populated area your water pressure is controlled by the municipal water system and is usually around 60 psi pounds per square inch.
If you think your hot water piping is clogged see clogged supply pipes hot water.
It is normal for building water pressure to vary between 20 40 psi or 30 50 psi depending on how your pump pressure control switch is set.
If your water is provided by a private well and the pressure or flow seem to vary or start and stop when you lose water pressure or lose all water supply for some time period but water pressure later returns or improves check the possible explanations outlined just below.
Unless you have a well and pump your household water pressure should remain constant but it may vary considerably when appliances showers sprinklers or other water using devices are engaged.
If the overall water level drops that will reduce your storage above the pump and often also reduce your recharge rate.