Hi there! Usually, two things can cause this:

1. The document was published to the portal with a layout that has since been unpublished in your Aspire system. Check and make sure it is published with your most recent, active layout. 
2. The opportunity or invoice is missing required fields - check and ensure the opportunity has an assigned sales rep, start date, and so on.

That’s great to know!!!

Last week, we have a client could not see his invoice in the portal. It turned out that the property that he associated with was marked as inactive. 

Then we reactive his property, he is able to review the invoice and pay it.