Hi Craniac,

Could be all the above. 9 gpm doesn't tell the whole story but is a start. You need to find out the pressure of the system. There are inexpensive pressure meters ($10 at HD) that screw on to your house connection point. This will be the system pressure of your house after your house pressure regulator in the garage. Your irrigation system can be connected before or after your house pressure regulator. I would guess with 16 heads your connected before the regulator and have street pressure which should be 60 to 80 psi. The house regulator limits this to 40-60 psi for appliances and faucet o-rings safety. Easiest is checking the heads for clogging. Unscrew irrig. head and look inside and clean. Let us know the system pressure and then we can help you more. You may have a faulty valve, clogged filter etc. Tell us about more details of your system. Thanks. Aloha

Remove the nozzle from the heads that are not working and see if water flows out at a decent rate. If it does look at the screw on the center of the nozzle it should be sticking up, if it looks like it is sunken in to the nozzle turn it to the left so it opens the flow.

Here is a link that might be useful: do_it_your_self_site

The average pop-up spray head uses 1-2 gallons per minute. Even with the minimum 1gpm the zone is seriously overzoned.I'm curious where you checked the flow. If it was after the solenoid valve for that zone, then you might have a bad valve or diaphragm. If the valve has a bleeder on it try opening the bleeder while in operation, if you see any pressure increase, you probably need to replace or rebuild valve. 9gpm is low even for municipal water supply. Generally you should get 12-15gpm from a 3/4" water meter at the meter or more. If it's a pump system the only pumps that pump that low are 1/2 or some 3/4 Horsepower jet pumps. If the other zones work and have the same or about the same amount/ type of heads, then you probably have a bad control valve. Good luck.