An alternative might be to collect the flow from the used nozzle for one minute or more in a container and then measure the volume. Calculate the flow rate in gallons per minute. Compare the discharge of nozzles at the beginning of the lateral to one at the end of the lateral.
A new nozzle could be installed and the system operated to collect discharge from it for one minute or more on the same lateral. Do this test at the beginning and at the end of a lateral. Compare these flow rates to get a percent difference. However, with worn nozzles in the irrigation zone, the pressure may not be what it should be and the results will not represent performance with all new nozzles.
Install new nozzles if the wear is excessive. New nozzles may bring the system closer to the original specs. Pump wear or other problems may be additional problems.
Nozzles may have been replaced by the wrong size nozzles at some previous time. Check nozzle specs or system specs to be sure the correct nozzle diameter is in place. The tests above should indicate a problem if the wrong nozzles are in use.
Drip emitters have been known to clog, leaving a plant without water. While new designs have reduced this problem, the drip emitter requires clean water and the emitter may clog from dirt or organic material in the water. Algae may be growing in the pipe lines and can be flushed to the emitter.