you can try turning them counter clock-wise but if you get less than one turn the trouble may be your system is already at it's max flow. if that's the case, then I'm sorry to say there is no simple fix.

He is correct. if you have experienced a drop in performance: check for leaks, leave the zone running for a long time and watch carefully. if the whole system is suffering, check the mainline for failed or corroded components.

one other possibility if the trouble is a single zone the valve itself could be the problem. sometimes a weak signal from the solinoid will keep the valve from fully opening.