Hi Sheri,
The most common reason for there being a difference is that you issued an invoice or took a cash receipt at the beginning of the month, but used "Retrieve to a specific period" when picking up the Ledger transactions. For example, if you issued an invoice on May 1, but retrieved and posted the transactions to April, then the AR amounts would differ.
I am assuming that you are comparing the AR Trial Balance to a recently printed Balance Sheet, just in case you had inadvertantly posted a back dated invoice/cash receipt.