Automatic Creation of Transaction Reference Number

In past, the journal and supplier transaction referral numbers were automatically created incrementally. But now both journal and supplier invoices are stuck and the number which is created automatically comes up with an error message that that number has been used before. So we need to input an unused reference number manually to create a transaction.

This may be relevant or not but the issue started at the start of September which is also our new financial year.

Any ideas?

Thanks

Helen