A common reason for a delay in processing income tax returns is the duplicate filing of a dependent. It could stem from a simple mistake, or from something much more nefarious or even criminal. The most important thing to know when you are searching for the person who claimed your child on their return is that the IRS cannot provide you with a name, but they can confirm that it was already filed and what steps to take next.
Per IRS guidelines, a dependent can only be claimed once per tax year. Translated, this means that each dependent can only be filed on one tax return per tax year. Two filings could trigger an audit, whereby the IRS requests verification of eligibility to claim the child from each filer.