This is most likely to be caused by the data being reconciled for one particular month (e.g. August) when they were transactions in a different month (e.g. April). To correct this you will have to unreconcile the transactions for that period and then reconcile them to the correct date/period.
Another reason for this issue could be a data issue. Sometimes the reconciled transaction will be in the correct period but certain details are missing in the back end of the system.
