I have numerous cards within a project that are not showing up within an active sprint on the Assignees boards. Some show up and others don't.... Any ideas what this could be

[Phill Fox]

The first place to check is that all the statuses are correctly mapped to columns on your boards. To do this on your board  click on the "board" prompt at the top right. Choose configure and then select columns. Make sure you have no unmapped statuses.

[Nic Brough -Adaptavist-]

Then check exactly what the board filter says, and all the quick-filters.  Also, are you definitely looking at the "work" view of the board (Agile hides certain issues in the planning view)?

[Megan Hallam]

It looks as though there are no unmapped statuses

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[Megan Hallam]

Nic I'm not sure what you mean. Could you please clarify?

[Phill Fox]

Do you see 13 issues in to do column, 1 issue in uat column and 15 issues in done column. If not then where does it differ? Have you any quickfilters turned on? This could restrict your view of the issues. 
If the numbers match and there are other issues you would have expected to see in the sprint board you need to check the filter for the board to understand why they are not being picked for display.

[Nic Brough -Adaptavist-]

What did I mean?  Pretty much what Phil just said.  Look at the filters you're using in the board.

On the last bit, if you are using a Scrum board, it has a planning view and a work view.  Which one are the issues missing from?