Hi aeesther
I would go back to basics with this one. Does the transaction appear in the Banking>>Find transactions window? Does the transaction appear in the General Ledger [Detial] report for that account?
Assuming that the transaction is listed in those reports, then you would be looking at seeing if the transaction has already been reconciled. To do this run the Account transactions report for that account and date range. You would then Customise that report to add in the Reconciled date. Does that transaction have a date listed? If so, it would indicate that the transaction has been reconciled and the date of that reconciliation that it occurred in.
If the transaction wasn't listed in those reports/windows, it would seem to indicate that the transaction has not been recorded in the software or has a different set of dates/amounts/accounts involved than first thought.
Should you still need some assistance with that getting screenshots of the Account transactions report for that transaction (including the reconciled date column) and one of the Reconcile accounts window would be required for further investigation.