Clogged line

There could be a clog in the lines feeding some of the sprinklers, or clogged up sprinkler heads.

• Remove all the sprinkler heads (the procedure will vary based on the type of heads).
• Inspect and clean the heads.
• Turn the system on.

If you don't notice any dirt or gunk coming out, and the pressure does not increase in the low flowing outlets. Cap the working outlets, and turn the system back on. If you still don't notice a difference, you might have to dig up the lines to inspect them.

Damaged line

The line feeding some of the sprinklers could have been crushed, damaged, or broken.

• Cap the working sprinklers.
• Turn the system on.

If the pressure in the low flow heads does not increase, you'll likely have to dig up the lines and inspect them.

Not enough pressure

It could just be that the system does not have enough pressure for all the heads to work simultaneously.

• Cap one of the working sprinklers.
• Turn on the system.
• Repeat.

If each time you cap an outlet you notice the pressure increase in the low flow heads, it could be that there is just not enough pressure to operate all the heads at once. In this case you might have to have the system split into zones, and only run one zone at a time.

Your pressure relief valve (let's stop using acronyms) is probably located on your water heater, it is actually a temperature/pressure relief valve. Sometimes a "pressure only" relief valve will be located on supply piping also. They are designed to open up and relieve pressure due to an unplanned temporary high pressure event, they both look something like this:

A pressure regulator is different. It is designed to regulate (control) your water utility company pressure before that pressure can damage your plumbing. This is the device you need go find. It should look something like this:

You will notice by a close look at the pictures that these two valves look very different.