If you've ever disputed with Transunion, you may have gotten a response letter from them stating something along the lines that they do not believe that your request was sent by you or a properly authorized 3rd party and that for that reason, they will not process your request. Don't worry, this is a stall tactic used by them to get you to stop your dispute process. And most people don't know what they should do after receiving this response from Transunion.
What you need to do is respond to the letter. This is why we have our clients mail their own letters and we do not mail them for the client. Although it wouldn't really matter since the bureaus tend to use this response no matter who mails the letter. You should also be sending all communications to the bureaus via certified mail with a return receipt.
When responding to the letter, you basically want to rebut their claim and put the burden of proof back on them. It's similar to requesting a method of verification. Only in this case you would be asking more for a method of explanation. You also want to make sure to include attachments of the original dispute letter and copies of your ID/Driver's License and proof of address (recent utility bill, bank statement or cell phone bill).
In response to this, I created a sample letter that you can use to respond to Transunion when they send you this response. Which, by the way, is an automated general response they send to everybody at some point. If you want the sample letter, you can purchase it below.