First, make sure nobody edited or deleted any AR subledger batches when they got to the GL. 

Second, check for GL-JE entries directly to the AR clearing account. 

Third, look for AR transactions using control accounts instead of revenue or cost accounts.

Thank you so much

And check if anyone reversed a batch in the GL only that had come from AR.  So instead of doing a credit note or reversing a receipt, they hit reverse batch in the GL and reversed there - it does NOT go back to the subledger and will always result in a difference to the subledger.

Hi,

Did you try printing the AR Aging by Posting Date to see if there is a difference between a document date in one month and a posting date in another month?