I couldn't get the Frequency subcommand to display blank values either.
But I was successful in using CTABLES. Try the following:
1. Analyze>Custom Tables>
2. RHS mouse click your variable of interest change Data measure to Nominal this is so you can see all of the values of the variable.
3. Drag the same variable into the rows of the Table GUI.
4. If you're still getting not getting the unused values within the CTABLES dialog box, make sure you doublecheck the Categories and Totals option and tick the Empty Categories.
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