Pop Up Sprinkler diagnosis help needed!

I have a 6 zone pop up sprinkler system. Five of the six zones work perfectly. The sixth doesn't I and I'm hoping you guys can help me figure out what's wrong. When the sixth zone is activated there doesn't appear to be enough water pressure to make the sprinkler heads pop up. There is water flowing in the pipes because water dribbles out of the unpopped sprinkler heads. I thought that I had a leak in the pipe because there's a significant saturated wet spot in one area of the lawn. However, I dug out the pipe everywhere around that spot and I there's no leak there. I'm thinking that the saturated wet spot is just from the water dribbling out of one of the sprinkler heads that isn't going up. I have no idea, however, why there isn't enough water pressure anymore for this zone. What could be the problem be from and what do I do to confirm the diagnosis? Thanks very much!!

I would have to say a plugged line, a bad solonoid valve or a broken pipe. Too bad it's all underground in most cases. That makes it more interesting to trouble shoot.

bob...

Bob--thanks for the ideas. What steps should I take to figure out which of those 3 possibilities is the problem? thanks!

Do you know where the solonoid valve is that controls that zone?

Find the valve for that zone and manually turn it on. If there's still a pressure problem, you may have an underground leak or you may have too many heads for that zone. Try removing one or more of the heads and capping the line off at that point. If you still don't have enough water pressure, the valve may be plugged and you'll need to dig it up and take it apart to clean it.