Look at the valve where the solenoid attaches. Using a very thin wire, make sure they are not blocked by debris. Try not to scratch the valve with the wire. With the top removed turn on the water and allow it to flow freely out of the bottom half of the valve to flush out any garbage. Check the rubber diaphragm for cracks and replace it if it is cracking or splitting.
Some valves have small screens inside to protect the ports.
Make sure they aren't plugged. Some valves have metering rods- long stainless steel rods that move up and down in a small hole. Make sure they move freely. Generally inspect the valve for anything else that looks wrong. There's not too much complicated stuff in there.