Make sure the items are all entered and marked returned, so that your Amount to Enter is zero on both Deposits and Checks.
If you need to enter internal transactions or corrections that are not on the statement, you may have identical non-zero amounts in Amount to Enter on both Deposits and Checks.
Make sure the 4-39 is posted for the month that you are balancing.
Run 8-28 to redo the closeouts for that month.
Use Alt+R on the G screen to repost the month.
Check the payroll posting for suspense account.
Check the 4-39 report for amounts with no category code.