To trouble shoot this issue, first remove the top and check for foreign objects and any rough spots on the valves interior walls.
Inspect the only moving part inside the valve called the “Stem and Disc Assembly” also check the “Cam” which is the plastic part at the top of the valve the plastic stem tip goes into with internal teeth that advance the disc to the next outlet.
Over time the spring inside the plastic stem wears out causing the valve to fail to cycle, the spring may still be strong enough to bounce the stem and disc up and down on the pin at the bottom of the valve however it may not be strong enough to fully engage the teeth on the Cam.
Also a worn cam even though it may look fine might have areas that have “flattened out” between the teeth that will cause the valve to stay stuck on one zone.
After the valve shuts down and water drains from the zone lines sometimes this can create a “vacuum lock” if air is not allowed to enter the valve to replace the water that has drained out. The 6000 series valve has an atmospheric vacuum breaker (AVB) built into the top of the valve to prevent this from happening. On 4000 series valves an AVB can be installed on the supply line before the valve if this is an issue.
In most cases replacing the Stem and Disc Assembly, Cam, and or installing an AVB on the supply line will get a valve that fails to cycle back in operation. For information on replacement parts, see INDEXING VALVE pages for replacements parts.