To ensure your documents aren't Finalized without the information you need from the signer, it's always recommended to designate important fields as Required . The Fields Manager has a checkbox to mark any field "Required" where a signer can't Finalize the document without completing the field first.
If you have a Sign Request that was Finalized, and you don't see a signature added, you can verify if this happened due to not having a Required signature field by checking the document's Audit page. Just add /audit to the end of the document URL, or click the button to Manage and Audit at the top right while viewing the Finalized Sign Request. 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 in Fields Manager:
To learn more about inserting and assigning fields, see: Create a PDF form by adding fillable fields to a document.
When a Sign Request has been Finalized with a Required signature, the Audit page will appear as below:
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