i,
Is the controller starting more than one zone at a time. How many zones are in the back? Was the sprinkler system installed by the house contractor and Is still under warranty? Check the common wire connections at the valves and controller only if you are beyond your warranty. Are both the front and back zones run by the same controller? Aloha

Only one zone is starting at a time. I have four zones in the back yard. The house is an old house and the system is not under warranty. I will check the connections again. You think this could be the problem? I have a separate controller for the front yard.

Thanks!

i,
It is a place to start. If the system is old, then I would redo the connections at the valves with waterproof screw caps and redo the connections at the terminal block of the controller. Then try to manually turn on each valve from the controller and see what happens. This advice is based upon that you have already tried manually turning on each valve by twisting the valve solenoids counterclockwise at the valves as you have stated. Post back what happens. Aloha

I redid all of the connections last summer but will try doing that again and seeing if it helps. I don't think I used waterproof screw caps though so that may be the problem. Thanks!