Ficus trees are very tempermental and don't like change. I know Southern California has had lots of unusual weather lately (the last 30 days) and some of the trees could be less hardy than others therefore showing signs of a problem.
Check for pests. Check for red spider mites using a piece of white paper held under a branch. Tap the branch and see if any tiny, tiny red specks show up on the paper.
Are the leaves sticky? This could be a sign of aphids/ants. I doubt that it is borers.
How are they being watered? If by drip are any of the lines plugged up?
County Extension will be able to diagnose the problem if you take in a branch of yellowing leaves. Then you will know bhow to treat the problem.
Liquid hand soap is not the answer to everything.