Just helped a customer with this.
There is no need to create a new project file or do a Pack & Go.
The issue is that when a Project's Folder Options > Content Center Files path = [Default], Showcase does not know how to find the Content Center Files.
The way Inventor works, if this propety has a value of [Default], then Inventor get's the location from Ribbon > Tools > Application Options > File tab > Default Content Center Files.
How to fix for End-User:
To test that the value held, select a different project from the list, then reselect the edited project. Expand the Folder Options and verify the full, default path is shown.
How Showcase should function (For Autodesk):
If Project file is using [Default]:
Well, this is quite anoting for the tube and pipe. All the elbow and Tee and all the good stuff from the content center disapear in showcase. What is the good if creating a workflow in the software if it's not working. Even when we read from Dhowcase and set the project file, its still not working. When are we going to have this fix?
If you follow the instructions in my post, then you should be seeing everything in Showcase. I just did a test on my machine and everything came through just fine.