Leak that just hasn't shown yet, tree root choking the pipe, issue with the valve that you cannot see, flow control closed part way. I'm leaning towards leak that hasn't shown itself yet.
Turn on a different zone that is comparable in size (volume output), and check the water meter to see how fast the dial is spinning, and how it sounds. Then do the same with your problem zone. If the meter sounds like the other zone without a problem, and is screaming like there is nothing impeding the flow of water, more than likely there is no obstruction, and the water is leaking out somewhere. If the dial is slowly moving and you can tell that the water isn't moving very fast, there is an obstruction in the valve or in the pipe. Sometimes the water will follow a trench (even if it is many years old) and surface out somewhere far away from the actual leak or break.