First of all you should check your G/L accounts used in your customer posting groups that they are not allowing direct posting.
Then you should check the G/L accounts used for A/R for manual transactions that should not be there.
If there is no transactions standing out i would suggest one of two approaches.
Either export the G/L Entries for the account to Excel and do the same with the Customer Ledger Entries. Sort them and sum them by posting date and compare them data by date to find the date the differences occur.
Or run the reconcile report for individual dates or span of dates until you are able to figure out what dates you need to reconcile.
Personally i prefer the Excel approach.
