hi dirk,

It is either physical (valves) or electrical(loose wire or broken wire. How old is your system of valves and controller? Do you have access to all the valves and are you familiar with the controller's electrical and the valve's solenoid connections. It is easy to research on-line. Make sure all wire connections are solid and secure in the controller and to the solenoid (just the ones not working). Turn on each valve manually with out the controller. Then turn on each valve manually using the controller. Tell us what happens. Aloha

Hey dirk, a lot of times I hear this, verbatim, the result is a bad module on your controller. Generally, most of the older controllers had only one or two modules, and sometimes one of them shorts out while the other still works..

Probably just need a new clock. $17.

For a complete check of sprinkler heads and valve check up, contact a professional sprinkler irrigation.

Here is a link that might be useful: Drip Irrigation

Agreed. If you're not too comfortable with these things, I'd call a pro. Its probably a simple fix.