When attempting to "Create Sales Invoice" from the Project Planning Lines, do you have the line you want to invoice as the active line? The process only runs for the line(s) you have selected. It could be that your page loaded with multiple Project Planning Lines and the line you want to invoice is not the active line. This could explain why it works from the Project Task function, because it would process through all Project Planning Lines within that Task.
As a test, you could open your Project Planning Lines, select all lines, then run the "Create Sales Invoice" process. Any Billable lines with a Quantity to Invoice should be transferred to the Sales Invoice.
Best Regards,
Ben Baxter
Accent Software Inc