The first thing I would check is to see if there was a broken lateral (the part of the piping between the valve and the heads).[V] You can check that by trying other zones.
Give more info so that we can visualize your installation;
Has the system ever worked properly?
Where is the supply piping connected to the water system? Before or after the tank?
When and if it has ever operated properly, does the pump cycle on and off or are you using a pumpstart relay?
What do you have to do to get psi back after shutting down system?[

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