Double check that Aging as of 11/30 still agrees with the balance sheet as of that date and make sure when you run Aging you are answering N to include Deposits.
 
If 11/30 is good and 12/31 is bad, check the GL Account for AR to see if anything posted to it in December that was a journal type other than RS (Sales) and CR (Cash Receipts).  Also you can run aging and trial balance for the AR account as of the 10th, 15th, 20th, etc. to identify the date it went out and look at the detail for that date to find what is different.