Hi,
In the product listing in backend, Is the sub element filtering set on "All sub elements" ? If not, please set this value.
If you think that there is an issue with your categories, you can go in the category listing, still in backend, and click on the "rebuild" button in the toolbar.
Regarding the import and the error for the prefix, I think that the issue is coming for the double underscore, the prefix value should be "yjdp_" instead of "yjdp__", so in the field please remove one underscore.