I would check that the account on record for direct deposit was entered into Social Security's computer correctly. Bring a voided check to your local office if you're able just in case.

With a very LARGE amount of backpay, they can need to have a higher up sign off before funds are distributed.

If you go into the local office, they can check if they had the correct account info.

You can always ask for a supervisor, even on the phone, at your local office. If you have a dire need, then let them know.