Do you really have a Visio 2017? I didn't know there was a version after Visio 2016.

It is possible your vssx stencil has become corrupted or can't be read. Try going to More Shapes > My Shapes > New Stencil and creating a stencil. Then add some shapes into that stencil and save it to the default location that Visio provides. Does that stencil then appear in the My Shapes flyout?

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Andrew Lockton
Chrysalis Design, Melbourne Australia