I think I have a blockage inside my water line. The water pressure to the shower, toilet, washer, etc. drops significantly when the toilet, washer, or sprinkler system is on. I've had a plumber out twice and a landscaper. They all focused on the bell valve on my sprinkler system because my sprinklers won't rise. Both the 25 year veteran plumber and the 8 year landscaper said they had never failed to fix a problem until now. The plumber did try to adjust the pressure on the incoming valve. The house is 10 years old. My neighbors have no problems. The pressure is 40-45 pounds where the water enters the house, which is the same as my neighbors. It must be a blockage inside the house. The pressure was fine the first few years, but has gotten worse. Assuming there is a blockage, how do I confirm and fix the problem. Help?!

40-45lbs of pressure is very low to begin with.

You can have fruther reductions in pressure based on backflow preventers being placed on the water supply like those required on a lawn irrigation system. If the pressure has dropped after the addition of the irrigation system this could be one cause.

Also, the pressure is derated depending upon how high above the main water supply line the top most water supply outlet is located.

The pressure is also reduced based on the developed length of pipe the water needs to run through to get to the fixtures.

Pipe sizing may also be wrong for the branches and you may not really have a pressure problem at all, but one of too little supply.

Unless you live in a region subject to hard water or other buildup in your lines which could restrict flow, you probably need to have your system assessed for proper sizing based on street pressure, height, length of pipes, demand from fixture units and pipe size to properly diagnose the problem.

Otherwise, a pressure pump added to you system could immediately increase the pressure.

Or, if you are on a well, have your pressure tank check. It or the bladder may have failed.

In our area pressure regulating valves are only required when incoming pressure is over 100# (most of the city). The regulators have a 25-75# range, but can't go more than the incoming and maintain volume. 40-45# incoming sounds very low. Try removing the valve and see if that helps, sounds like it's not needed anyway, then do all the stuff homebuild said.