Although there may be several reasons why water pressure is decreasing, in most cases water leaks or mineral build-up within your plumbing lines cause you to experience this problem. It is encouraged you check if any inhouse filtration is plugged, bypass the softener to see if it is plugging and check to ensure valves within the house are fully open and not leaking.
Most Likely Causes: (Homeowner or their plumber can check)
Plugged whole house filter – homeowner or their plumber can check, replace clean or bypass
Plugged softener – homeowner or their plumber can check or bypass
Valve within house does not open all the way – homeowner or their plumber can check
Plugged individual faucet, filter or screen – if only an individual location has low flow
If irrigation related – could be a lot of reasons (leaks, too many heads on a zone)