Question
one of the head in a zone (5 heads total in this zone) does not rise.  The rest of 4 heads works ok.
Thanks.

Answer
Hey zlo, more than likely you have debris down in the bottom of the head, or in the nipple below the head, or the fitting below the head. What you can do is dig the dirt from around the head. Below the head you will notice a nipple, and below the nipple you will see a fitting that the nipple is screwed into. What I would suggest, is once you dig the dirt from around the head and nipple down to the fitting, simply unscrew  the head from the nipple first, but not from the fitting. Now turn this zone on, and flush the debris from the fitting and nipple. Also check the bottom of the head and make sure there is no debris in the bottom of the head. If when you turn this zone on, you don't get a steady stream of water shooting out from the nipple, then you still have debris or a blockage further down, either in the nipple or the fitting. If you still have a blockage, next unscrew the nipple and examine the fitting and see if you see debris in the fitting. Sometime you might have to put a screwdriver or piece of wire into the fitting to get the debris loose. If you still don't get a good flow of water and the fitting and nipple are clean of debris, then the problem might be further up the pipe, and you might have to dig along the pipe and look for another fitting (elbow, tee, etc.) that could have a blockage in it. Start with the head , nipple and fitting and see what you can find. Good luck and have a great day. Happy Easter.