There are many reasons for low flow. When working on the plumbing it is very easy to accidentally introduce small stones and other crud into the system. This stuff finds its way into valves, filters and waterlines. Enough of the right-sized stuff can cause low flow.
Take valves and other components apart. Clean them and/or replace filters. Don’t accidentally let in more contamination when doing this.
Here is a different but equally annoying dirt symptom: If you are using a manifold (one pipe feeding multiple valves), sometimes crud trapped in one valve can cause water to flow to its zone when other zones water. I fought this for weeks only to discover that there was a pea-sized stone in one valve that would hide just out of sight in the valve until watering, at which point it would roll out and push on the valve’s diaphragm. When watering stopped the stone rolled back out of sight.