It's always recommended to designate any important fields as Required to ensure that the document isn't Finalized without the information you need from the signer. The Fields Manager has a checkbox to make any field "Required" where a signer can't Finalize the document without completing the fields first.
If you have a Sign Request that was Finalized, and you don't see a signature added, you can check the Audit page to see whether or not they've Finalized without completing a signature field. Just add /audit to the end of the URL of the document to view its Audit page, or at the document URL, click the button to Manage and Audit at the top right. Then, look in the Assigned Fields section under that signer to see if they have any values for signature/initial fields. Any Required fields will also have a lock icon in the Req. column.
Here is how the Audit page will appear if the signer Finalized your document without completing a signature field:
If that's how the Audit page appears for your signer, you can simply resend the document to them with a Required signature field. Just make a copy of the document on the Audit page using Document Actions > Make a Copy and then use the Fields Manager to make sure any signature fields are checked as Required and resend it as a new Sign Request. Here's how to designate a field as required: