I recommend verifying that your dependent's last name and social security number is listed on your Tax Return exactly as it appears on your actual social security. I suggest locating the actual social security card, not looking at a previous tax return or other documents to verify.
If a dependent has two last names or two middle names, that could be causing the issue.
If there are two last names, use the "first" last name, then leave a space and include the last name.
If there are two middle names, use the "second" middle name as the last name, then leave a space and include the last name.
Also be sure to include any suffix to the last name like Jr, Sr, III, etc.