Lawn Sprinkler's water pressure is low in one zone
Manually operate the valves until you find the zone that isn't working right. It might be a solenoid, but usually those are an &quot;all or nothing&quot; failure. Solenoids can fail electrically or mechanically. The coils can short out and/or the plunger itself can get stuck.  Once you find the failing zone you can further diagnose what's going on with it. And we can help better with more info.
Co-incidentally I have a zone valve right now that is stuck open.  So I had to shut down the supply. Probably fix it tomorrow.  Almost certainly a stuck solenoid.
Pictures and more information on the zone valves will enable us to help you better.
You are lucky, IMHO, to get 20 years out of zone valves without service.  They must have been pretty high quality to start with.