Hi DePez
AccountRight uses an invoice number sequence which is based on the last recorded transaction in the system. For example, if you record invoice number 00000010 the next invoice would be invoice number 00000011 then 00000012, 00000013...
The invoice number autonumber sequence can get pulled out of sequence if transactions are deleted or transactions are recorded out of order. To fix that situation you need to go into the invoice number field on the required sale and update the invoice number before recording that invoice. This process may need to be completed multiple times to ensure that AccountRight picks up on the latest invoice number sequence.
If you haven't already, turning on Setup>>Preferences>>Sales>>Warn for duplicate Invoice Numbers on Recorded Sales can help if you are having trouble with older invoice numbers. This will warn the user that an Invoice number has already been used by the system (if you attempt to record an invoice with the same number) and thus is can give the user an indication to correct the invoice number so the autonumber doesn't go out of sequence.
Also you can try to clear the cache at that often solves odd software behaviour.