So you've done your diligence and confirmed your business defendant's structure, correct name and what address they registered with the government. But when the process server shows up with your summons, they're gone.
Or maybe they use a mailbox rental for an address, like a UPS Store location. That's fine for substitute process service, until they close their mailbox account.
Just because a business registered an address with the CA Secretary of State (SOS) or their county clerk, doesn't mean the company is still operating there, if they're operating at all, or if they ever really operated there. Businesses and their agents sometimes move and fail to update that information with the government.
So where do you look now?
Corporations and LLC/LPs
The best place to start once again, is the SOS business search tool. When you pull up the company's listing, notice the PDF documents under the listing. These are the registration and statement of information forms the company has filed. Officer and member names can appear in those filings. Sometimes they've used their home addresses. Tell the process server to try those addresses. For good measure, run the names in whatever proprietary database you use, or hire a private investigator to nail down their whereabouts.
Sole Proprietors
These are "individual" defendants so you can only search for that one person. If they use a fictitious business name and aren't found at the address they registered with the county clerk, run them in your database or hire a skip trace investigator to help.