First of all, I'm just a DIY home owner like you (the others posting here are a little more familier with the industry that I am).
Having said that, I don't think your problem is the valve per-sae. I've never heard of a valve going bad such that it caused a reduced flow to a zone. About the only way I can think such an issue could occur is if something has clogged the valve. But even that seems unlikely because the typical way valves get clogged is a small piece of something gets stuck in the piolet hole and it keeps the valve from either opening or closing completely. So I'm guessing that if the problem is a clog at the filter, you might be able to fix the problem by removing the guts of the filter and flushing the line from the water source to the manifold with the guts removed.
Now if the problem is stuff in the line, and you're having a problem with even the 1st head, then that's the only head you need to open to flush the line.
On a side note, I seem to recall reading that FL has a lot of sandy soil, and as such, it could take a long time (and possibly never) for the soil to become soggy if there is a break in the line. I've even read stories where a break was NEVER found because of surface water. In one of these situations, a break occurs say near a french drain or something, basically somewhere that the leaking water is able to flow away from the break and never build up enough to cause a puddle at the point of the break.
Random thought... is there an automatic drain that perhaps is not properly closing when the system gets pressurized?
But other wise, the most likely source I keep coming back to is a broken water line. If you can flush the system from the source to the valve, then from the valve to the 1st head, and you still have a problem, then it sounds like a broken water line to me... and of course the break could be ANYWHERE (before the 1st head, near the last head, anywhere since the a break anywhere will keep the whole circuit from pressurizing properly).
OK, another random though to try. Use your water meter to compare the flow of water between the "bad" circuit, and one of the "good" circuits of similar size. Do something like make sure no one is using ANY water in the house (maybe even shut off the main to the house). Then turn the bad zone on for say 10 minutes and measure how much water flowed... or measure how long it takes for the system to use 10 gallons. Then do the same for one of the "good" zones. If the problem is a clog in the line, then the flow to the "bad" circuit should be much lower than a "good" circuit. If the problem is a break in the line, then the flow to the "bad" circuit should be much higher than a "good" circuit. But remember, you have to base that on a zone that is about equivilent.