I don't know the answer to this but am bumping it to see if someone else does this morning.

the trouble is most likely a loose connection along the common wire. as for finding your missing valves it will probably take someone with a feel for how systems are laid out to probe for them or doing a wire trace of the common wire which will also find where your loose connection is.

I replaced a solenoid earlier today and that did not fix the problem. We are selling this house so I don't want to put a lot of money into fixing it, but I just hate not being able to figure out what the problem is.

all i can suggest at this point is to go to each valve box you know of and redo all the wire connections and see if that changes anything. if not then the trouble is going to be in a box you have been unable to find yet.

Ok, you said the controller is producing Approx 24 Volts to each solenoid.

Your next step with a Voltage meter is to set it to omes. with the black "prong" on the common, and the red prong on each zone wire you should get readings between 30, and 55(60). If your reading is 0.0 or open line you typically have a cleanly cut wire. A high reading (typically 100+) is usually some kind of damage on the wire to include a connection as mentioned above. A reading below 30 is typical of a solenoid problem.

If its wire Im sorry for you, time is money, and even with a locater it can take a while to find the problem area. In My opinion, if the gardeners just planted; start there.