The invoice number that the order will take comes from an internal Dimensions control table. When it retrieves this number Dimensions will also check to see if that invoice number has been already entered onto the ledger (say from the sales ledger directly) and will skip this invoice number if it finds that it has already been used.
The expected invoice numbers may have been used for another customer but the assigned invoice numbers can be found by looking in the ledger for transactions against that customer.
You may also see invoice numbers being missed if an invoice is generated but not accepted. When a sales order is invoiced, the system allocates the next number in the sequence. If the invoice is not fully accepted at the time, then the number will be held until the invoice is completed. An example of this would be:
You invoice a sales order and accept and post the invoice - this gets invoice number one
You invoice a sales order but do not accept the invoice when it is generated - this gets invoice number two
You invoice a sales order and accept and post the invoice - this gets invoice number three
When you review your printed and completed invoices they will appear to jump from one to three, missing two.