Make sure the Infopath field is not marked as "hidden" in K2 studio. Any XML or Process fields that are marked as "hidden" will not show up in the reporting. If it is hidden you can uncheck it and re-export but you will probably have to run a new instance of the process to see the data. I am almost positive that it will not un-hide the data for process instances that ran on the previous versions of the process.
-Eric
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