To repair a sprinkler when the problem is low water pressure, make sure that the valves on the backflow mechanism are opened and that the handles on the valves are parallel with the backflow pipe. If that doesn't work, inspect the ground above the water line for signs of leaks. Dig out the pipe, shut the irrigation system off, and repair the pipe by cutting out the broken piece of pipe and inserting a slip coupling instead.
If you didn't detect any leaks, the line might have collapsed. Start digging up the pipe after the last sprinkler head that is working correctly. If there is a tree growing next to the line, you might want to start digging there, as the tree's roots might have wrapped around the pipe and damaged it. Once you find the problem, repair the pipe as you would repair it if it was leaking. If a tree damaged the line, relocate the pipe to run further away from the tree.
One of the reasons a separate sprinkler head might not be working is that it may be clogged with dirt. To fix that, take the head out of the riser, take it apart, and wash all parts thoroughly, paying extra attention to cleaning the plastic screen basket. Reassemble the sprinkler head. Adjust the sprinkler to water a specific area.