It may be that this candidate's supplier is set to a different processing mode by mistake. You should check the Supplier Entry record to see whether this supplier is set to 'Self-Bill' or not; if you are expecting to see their payment waiting in the Purchase Invoice processing mode and this supplier is set to self-bill that would explain it (and vice versa).
The Supplier can be switched to either purchase invoicing or self-billing by ticking/removing the tick from the Self Bill box within Supplier Entry. Any timesheets that are waiting to be paid will then move into the correct processing area.
We also recommend you use the Unprocessed Supplier Payments report within the Supplier Payments menu as part of your regular process to identify any supplier payments not yet processed.