If you think the clerk is incorrect, you can submit an application for an order directing the clerk to enter the default. Before you do that, have an attorney review the proof of service.
Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
The Clerk's office has a "reject sheet" that they typically mark up and enclose with any items rejected. If you find that sheet, it should give you a good indication what you may need to cure. Otherwise, you can pay a visit to the Clerk's office and see if they can explain the defect to you. If it turns out that the "proof of service" form was defective, such as an outdated form was used, the remedy would be to ask your process server to fill out the correct form and submit it. However, if "service" was not properly accomplished, you will likely have to pay your process server to make proper service and provide you proof. If personal service is made, you will need to allow at least 30 days before you can submit another request for entry of default. Be sure to consult your own attorney to protect your legal rights.