Legal Question in Real Estate Law in California

"The defendant has not answered the claim." Do you mean he did not file an Answer to the Complaint, or to the form interrogatories in DISC-001? Assuming you mean the interrogatories, your manner of service (as shown in the proof of service) must describe how you served him. I'm assuming he and you do not have an attorney, so then contact him and find out. He might genuinely not have received them. If you believe, instead, that he did receive them but is lying, then you would have to file a motion to compel answer in court. But that will be complicated and has procedural requirements, and therefore, I highly recommend you obtain an attorney when it gets into litigation like this.

Rather than file any motion, you can simply re-serve the Form Interrogatories. This time, send two sets, one by regular mail and one by certified mail, return-receipt requested. After a few days, contact the defendant or his attorney and ask if they were received this time. This is almost as fast as filing a motion, because the motion must be filed and served well before a hearing, and even if you win at hearing, the judge will give the defendant time to respond with his answers.

I might add to my previous answer that if all else fails, you can use a professional process server or the sheriff to serve process for a reasonable fee; most people won't lie about receipt of papers served by a third-party pro, especially a sheriff.