I've found that it's either a broken pressure regulator or a valve isn't turned on all the way when I run across your situation. Sometimes a plumber wont turn the water back on correctly at the water meter.
One time on a 2 inch line they had 2 bad ball valves, one on either side of the backflow preventer. Both balls were broken inside. The valve handles looked open but inside they were barely open reducing the pressure to the whole sprinkler system.