It might because your export job start time (the "Export from" item in export create/edit page) is set too early and the first one or few runs were not getting any events. You can manually run the job a few times (by clicking the 'run' link in the job list page) and see if you get any event exported.
You can also check the HadoopConnect.log under $SPLUNK_HOME/var/log/splunk/. Each time export is run, you will get two lines like this:
2014-04-14 13:20:14,868 INFO run_export.py [main] [878] - search:search (_indextime=1397494147 OR _indextime=1397494148 ... OR _indextime=1397495946) host=foo| eval _dstpath=strftime(_time, "%Y%m%d") + "/" + strftime(_time, "%H") + "/" + host | fields _dstpath host source host,source,sourcetype | dump basefilename=foobar ... exportname="Host"
2014-04-14 13:21:51,304 INFO run_export.py [renameTmpFiles] [354] - Renaming 2 temporary files in HDFS...
You can run the first part of the search query in search bar and verify if there is any events fall into that export time frame:
(_indextime=1397494147 OR _indextime=1397494148 ... OR _indextime=1397495946) host=foo| eval _dstpath=strftime(_time, "%Y%m%d") + "/" + strftime(_time, "%H") + "/" + host | fields _dstpath host source host,source,sourcetype
The second log statement tells you how many event files are exported to HDFS.
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