First check out this previous post which has a good list of items to check:
I would say start with the FixVersion / AffectsVersion fields and make sure they are not set to hidden, as the issue will not show up if either of these fields are hidden.
Then are the issues you are attempting to import still unresolved and open or are they closed? As Portfolio for JIRA is a forward looking planning tool and the default filtering inside a plan is set to filter out done issues that are older than the current sprint. Additional details on the filter options can be seen in "Using the scope view", and expanding the "Status" and/or the "Completion date" range to include additional issues outside the default range. These two are located under the More dropdown after creating the plan.
Next, for a referance point, Look at is the documentation for "Configuring the issue sources". How are your sources configured (Project, Board, or Filter?), and if you're using a board or direct filter the Post above already notes checking filter permissions, but also what is the filter in use, and does it include the issues you are expecting to see in the plan (exe, are you possibly excluding an issue type)?
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