If you're looking for an invoice and can't find it in your Bills list, here are the most common reasons.
Invoices in pending payruns
Invoices included in a pending payrun are removed from the Bills list to prevent double payment. This is expected behaviour.
To find these invoices, go to Bills > Payments > Pending to see all invoices currently in pending payruns. From there, you can view the invoices, edit the payrun, or complete or decline the payment.
Making a partial payment on an invoice in a pending payrun
If you need to make a partial payment on an invoice that's already in a pending payrun, complete that payrun first. Once it's processed, the invoice will reappear in your Bills list with the remaining balance, and you can create a new payment for the rest.
For more detail, see Pay part of an invoice.
Also worth checking: If the invoice has already been paid, it will have moved to Bills > Payments > Paid.