The question of missing invoice numbers could be asked in two different ways:
- I have posted an invoice and it is no longer in history.
- I am looking at history and all the invoices are not exactly sequential, there are a few missing.

The answer to your question depends on which of these questions you are asking.

1) If it was an invoice number that was posted but missing in history, first check to see if that invoice number appears in Receivables, either in current or historical transactions. This could help identify whether the invoice made it to history but was subsequently deleted. Do you still have your posting journals unpurged? If so, preview the entire list and use the search field at the top to look for that invoice number.

2) If you just happened to notice that all invoice numbers in a certain range are not there, perhaps that invoice number was never posted. Perhaps an invoice was created but the document type was changed to a credit note; in this case the 'invoice' number is lost. Or perhaps that an invoice was deleted during batch entry and re-created, but because numbers are assigned sequentially the 'deleted' invoice was skipped.

You may also want to make sure that the Data Integrity Check runs clean.