CSV Import won't import previously imported issues even if they've been deleted

I set up a standalone JIRA Demo and imported some issues (about 8000 of them). I wasn't happy with the fields I'd set up and the way I'd munged my source data to fit into JIRA's fields, so I did a Bulk Operations and deleted all the issues in the database. Then I edited my source and re-imported the data. No issues were created! This seems like a bug to me. I've checked the support forums and can find no other mention of this behaviour.

Even though my All filter shows no issues, if I open the jiradb.script database file with a text editor, I can see all of the old issue data in there. Shouldn't that all be deleted when the issues are deleted? If not, shouldn't the CSV imported be able to re-import those issues?

thanks
S