If you have double checked in Matter Status that the Invoice appears open and you still do not see the Invoice in the application window (and you are sure you are on the correct Client-Matter)  you need to open a support ticket with Orion Support.

This could be a symptom of a process that may have stopped in Orion and failure of a data file to update with data. It is unusual and should be reported to Orion Support. There may be a data issue that needs correcting. This can be caused by workstation, hardware, network, or database issue that is external to the Orion program.

Orion Support will need the Invoice Numbers and Client-Matter Numbers that are having the issue.  Screenshots of any outstanding payment records that may be under Action > Payment Application Posting that are relevant will be needed.

Orion Support may have to use a special utility to edit the data files to correct the issue. Note that editing or correcting data files may result in billable charges as this is not considered covered under your maintenance and support agreements. See the article entitled What is Considered Billable under the Orion Support Contract.