There are two possible reasons for this:
1) The invoice was not confirmed by you in time for the payment to be generated. In such cases, the job(s) will be included in the next payout.
2) There is a dispute with a customer, e.g. due to a complaint, which needs to be resolved prior to the release of the payment.
Payout cut-off is always the end of day on the 15th and last day of the month.