Low water pressure causes more water waste and a higer utility bill because each activity calls for you to leave the water running longer.  The most common causes of low water pressure include:

• Clogged Pipe:  A clogged water pipe can contribute to low water pressure.  Attempting to unclog a water line yourself could lead to water contamination in your home.
• Faulty Pressure Regulator:  Every faucet in your home will be affected if the pressure regulator attached to the pipes outside your home fails.
• Mineral Buildup:  Overtime, your pipes, faucets, and shower heads will be affected by mineral buildup.  If the change in your water pressure happened gradually rather than all at once, having the buildup professionally removed could fix the water pressure.
• Old Pipes:  Old iron pipes are especially susceptible to corrosion and can inhibit the flow of water.  These pipes corrode from the inside out, so it would be impossible for you to identify this as the problem without a camera inspection from a plumber.

It may seem unlikely, but invasive tree roots can cause leaks in your plumbing, resulting in a high water bill and low water pressure.  Your pipes can suffer extensive damage from invasive tree roots if the problem is not handled right away.

The usual reason is that the flap in the bottom of the tnak isn’t sealing shut.  Take the top off the flush tank and flush the toilet whie watching the rubber flap.  Does it completely close?  If not, even though water is entering the tank, some of it is flowing out through the unsealed flap and therefore, the water continues to run because the float ball trigger is never activated to stop the flow since water never reaches the fill line.  That’s why jiggling the handle works to stop the running water.  It’s attached to the rubber flap with a chain, and jiggling it will reposition the flap so that it may seal more securely.